Issue:
Bug: [FEATURE] Add support for gauge metrics and include IMC metrics



Add support for gauge metrics. Additionally also add a background thread, which will be spawned during application startup, that runs for every fixed interval (configurable) of time, collecting the metrics. For now this collector can collect the gauge metrics of `cache_entry_count` of all the cache types. 

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR adds support for gauge metrics. Additionally the pr also adds a background thread, which will be spawned during application startup, that runs for every fixed interval (configurable) of time, collecting the metrics. For now this collector collects the gauge metrics of `cache_entry_count` of all the cache types. 

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [x] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
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Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

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can focus on the implementation (unless it is an obvious bug or documentation fix
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## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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Hit list payment methods for a merchant endpoint and check for `router_CACHE_ENTRY_COUNT` metric in grafana

![image](https://github.com/juspay/hyperswitch/assets/70657455/7b08c4e5-d7f8-433e-8561-6bdefef039dc)

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:


Hit list payment methods for a merchant endpoint and check for `router_CACHE_ENTRY_COUNT` metric in grafana

![image](https://github.com/juspay/hyperswitch/assets/70657455/7b08c4e5-d7f8-433e-8561-6bdefef039dc)

Patch:
diff --git a/config/config.example.toml b/config/config.example.toml
index f4a6a66cd61..81516ee8b6b 100644
--- a/config/config.example.toml
+++ b/config/config.example.toml
@@ -112,6 +112,7 @@ otel_exporter_otlp_endpoint = "http://localhost:4317" # endpoint to send metrics
 otel_exporter_otlp_timeout = 5000                     # timeout (in milliseconds) for sending metrics and traces
 use_xray_generator = false                            # Set this to true for AWS X-ray compatible traces
 route_to_trace = ["*/confirm"]
+bg_metrics_collection_interval_in_secs = 15           # Interval for collecting the metrics in background thread
 
 # This section provides some secret values.
 [secrets]
diff --git a/config/deployments/env_specific.toml b/config/deployments/env_specific.toml
index a097e1b66dc..9ab790b8ee8 100644
--- a/config/deployments/env_specific.toml
+++ b/config/deployments/env_specific.toml
@@ -139,6 +139,7 @@ otel_exporter_otlp_endpoint = "http://localhost:4317" # endpoint to send metrics
 otel_exporter_otlp_timeout = 5000                     # timeout (in milliseconds) for sending metrics and traces
 use_xray_generator = false                            # Set this to true for AWS X-ray compatible traces
 route_to_trace = ["*/confirm"]
+bg_metrics_collection_interval_in_secs = 15           # Interval for collecting the metrics in background thread
 
 [lock_settings]
 delay_between_retries_in_milliseconds = 500 # Delay between retries in milliseconds
diff --git a/config/development.toml b/config/development.toml
index 2c6d67e7515..e475459573a 100644
--- a/config/development.toml
+++ b/config/development.toml
@@ -10,6 +10,7 @@ log_format = "default"
 traces_enabled = false
 metrics_enabled = false
 use_xray_generator = false
+bg_metrics_collection_interval_in_secs = 15
 
 # TODO: Update database credentials before running application
 [master_database]
diff --git a/config/docker_compose.toml b/config/docker_compose.toml
index 2a4fe369286..2cd93eade01 100644
--- a/config/docker_compose.toml
+++ b/config/docker_compose.toml
@@ -18,7 +18,8 @@ traces_enabled = false                                      # Whether traces are
 metrics_enabled = true                                      # Whether metrics are enabled.
 ignore_errors = false                                       # Whether to ignore errors during traces or metrics pipeline setup.
 otel_exporter_otlp_endpoint = "https://otel-collector:4317" # Endpoint to send metrics and traces to.
-use_xray_generator = false
+use_xray_generator = false                                  # Set this to true for AWS X-ray compatible traces
+bg_metrics_collection_interval_in_secs = 15                 # Interval for collecting the metrics in background thread
 
 [master_database]
 username = "db_user"
diff --git a/crates/router/src/bin/router.rs b/crates/router/src/bin/router.rs
index 094f799cb27..8271e458b24 100644
--- a/crates/router/src/bin/router.rs
+++ b/crates/router/src/bin/router.rs
@@ -2,6 +2,7 @@ use router::{
     configs::settings::{CmdLineConf, Settings},
     core::errors::{ApplicationError, ApplicationResult},
     logger,
+    routes::metrics,
 };
 
 #[tokio::main]
@@ -27,6 +28,11 @@ async fn main() -> ApplicationResult<()> {
 
     logger::info!("Application started [{:?}] [{:?}]", conf.server, conf.log);
 
+    // Spawn a thread for collecting metrics at fixed intervals
+    metrics::bg_metrics_collector::spawn_metrics_collector(
+        &conf.log.telemetry.bg_metrics_collection_interval_in_secs,
+    );
+
     #[allow(clippy::expect_used)]
     let server = Box::pin(router::start_server(conf))
         .await
diff --git a/crates/router/src/routes/metrics.rs b/crates/router/src/routes/metrics.rs
index 1123be1a874..18014c6e193 100644
--- a/crates/router/src/routes/metrics.rs
+++ b/crates/router/src/routes/metrics.rs
@@ -1,4 +1,8 @@
-use router_env::{counter_metric, global_meter, histogram_metric, metrics_context};
+pub mod bg_metrics_collector;
+pub mod request;
+pub mod utils;
+
+use router_env::{counter_metric, gauge_metric, global_meter, histogram_metric, metrics_context};
 
 metrics_context!(CONTEXT);
 global_meter!(GLOBAL_METER, "ROUTER_API");
@@ -133,5 +137,5 @@ counter_metric!(ACCESS_TOKEN_CACHE_HIT, GLOBAL_METER);
 // A counter to indicate the access token cache miss
 counter_metric!(ACCESS_TOKEN_CACHE_MISS, GLOBAL_METER);
 
-pub mod request;
-pub mod utils;
+// Metrics for In-memory cache
+gauge_metric!(CACHE_ENTRY_COUNT, GLOBAL_METER);
diff --git a/crates/router/src/routes/metrics/bg_metrics_collector.rs b/crates/router/src/routes/metrics/bg_metrics_collector.rs
new file mode 100644
index 00000000000..65cb7a13e5e
--- /dev/null
+++ b/crates/router/src/routes/metrics/bg_metrics_collector.rs
@@ -0,0 +1,46 @@
+use storage_impl::redis::cache;
+
+const DEFAULT_BG_METRICS_COLLECTION_INTERVAL_IN_SECS: u16 = 15;
+
+macro_rules! gauge_metrics_for_imc {
+    ($($cache:ident),*) => {
+        $(
+            {
+                cache::$cache.run_pending_tasks().await;
+
+                super::CACHE_ENTRY_COUNT.observe(
+                    &super::CONTEXT,
+                    cache::$cache.get_entry_count(),
+                    &[super::request::add_attributes(
+                        "cache_type",
+                        stringify!($cache),
+                    )],
+                );
+            }
+        )*
+    };
+}
+
+pub fn spawn_metrics_collector(metrics_collection_interval_in_secs: &Option<u16>) {
+    let metrics_collection_interval = metrics_collection_interval_in_secs
+        .unwrap_or(DEFAULT_BG_METRICS_COLLECTION_INTERVAL_IN_SECS);
+
+    tokio::spawn(async move {
+        loop {
+            gauge_metrics_for_imc!(
+                CONFIG_CACHE,
+                ACCOUNTS_CACHE,
+                ROUTING_CACHE,
+                CGRAPH_CACHE,
+                PM_FILTERS_CGRAPH_CACHE,
+                DECISION_MANAGER_CACHE,
+                SURCHARGE_CACHE
+            );
+
+            tokio::time::sleep(std::time::Duration::from_secs(
+                metrics_collection_interval.into(),
+            ))
+            .await
+        }
+    });
+}
diff --git a/crates/router_env/src/logger/config.rs b/crates/router_env/src/logger/config.rs
index 09d285a8628..135451f266e 100644
--- a/crates/router_env/src/logger/config.rs
+++ b/crates/router_env/src/logger/config.rs
@@ -103,6 +103,8 @@ pub struct LogTelemetry {
     pub use_xray_generator: bool,
     /// Route Based Tracing
     pub route_to_trace: Option<Vec<String>>,
+    /// Interval for collecting the metrics (such as gauge) in background thread
+    pub bg_metrics_collection_interval_in_secs: Option<u16>,
 }
 
 /// Telemetry / tracing.
diff --git a/crates/router_env/src/metrics.rs b/crates/router_env/src/metrics.rs
index e145caace00..e75cacaa3c9 100644
--- a/crates/router_env/src/metrics.rs
+++ b/crates/router_env/src/metrics.rs
@@ -101,3 +101,22 @@ macro_rules! histogram_metric_i64 {
         > = once_cell::sync::Lazy::new(|| $meter.i64_histogram($description).init());
     };
 }
+
+/// Create a [`ObservableGauge`][ObservableGauge] metric with the specified name and an optional description,
+/// associated with the specified meter. Note that the meter must be to a valid [`Meter`][Meter].
+///
+/// [ObservableGauge]: opentelemetry::metrics::ObservableGauge
+/// [Meter]: opentelemetry::metrics::Meter
+#[macro_export]
+macro_rules! gauge_metric {
+    ($name:ident, $meter:ident) => {
+        pub(crate) static $name: once_cell::sync::Lazy<
+            $crate::opentelemetry::metrics::ObservableGauge<u64>,
+        > = once_cell::sync::Lazy::new(|| $meter.u64_observable_gauge(stringify!($name)).init());
+    };
+    ($name:ident, $meter:ident, description:literal) => {
+        pub(crate) static $name: once_cell::sync::Lazy<
+            $crate::opentelemetry::metrics::ObservableGauge<u64>,
+        > = once_cell::sync::Lazy::new(|| $meter.u64_observable_gauge($description).init());
+    };
+}
diff --git a/crates/storage_impl/src/redis/cache.rs b/crates/storage_impl/src/redis/cache.rs
index e4efab0da33..6ad96be2795 100644
--- a/crates/storage_impl/src/redis/cache.rs
+++ b/crates/storage_impl/src/redis/cache.rs
@@ -207,6 +207,16 @@ impl Cache {
     pub async fn remove(&self, key: CacheKey) {
         self.inner.invalidate::<String>(&key.into()).await;
     }
+
+    /// Performs any pending maintenance operations needed by the cache.
+    pub async fn run_pending_tasks(&self) {
+        self.inner.run_pending_tasks().await;
+    }
+
+    /// Returns an approximate number of entries in this cache.
+    pub fn get_entry_count(&self) -> u64 {
+        self.inner.entry_count()
+    }
 }
 
 #[instrument(skip_all)]