Issue:
Bug: [FEATURE] log "MERCHANT_ID_NOT_IN_FLOW" if merchant_id is not found



### Feature Description

Here,
https://github.com/juspay/hyperswitch/blob/9b92d046de9fb794d67163582af4360d5e558037/crates/router/src/services/api.rs#L729

Instead of `unwrap_or("")`,  

### Possible Implementation

Change it to some const `MERCHANT_ID_NOT_FLOW`.

### Have you spent some time checking if this feature request has been raised before?

- [X] I checked and didn't find a similar issue

### Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

### Are you willing to submit a PR?

No, but I'm happy to collaborate on a PR with someone else

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [x] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
In case the merchant_id is not found from the request, instead of using an empty string `("")` as merchant_id for traces and metrics, use `"MERCHANT_ID_NOT_FOUND"`

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] 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.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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Fixes #2153 

## How did you test it?
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Or did you test this change manually (provide relevant screenshots)?
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## Checklist
<!-- Put an `x` in the boxes that apply -->

- [ ] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [ ] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable


Testing:

Patch:
diff --git a/crates/router/src/services/api.rs b/crates/router/src/services/api.rs
index 99400050df3..6c116e995ce 100644
--- a/crates/router/src/services/api.rs
+++ b/crates/router/src/services/api.rs
@@ -726,7 +726,10 @@ where
         .authenticate_and_fetch(request.headers(), state)
         .await
         .switch()?;
-    let merchant_id = auth_out.get_merchant_id().unwrap_or("").to_string();
+    let merchant_id = auth_out
+        .get_merchant_id()
+        .unwrap_or("MERCHANT_ID_NOT_FOUND")
+        .to_string();
     tracing::Span::current().record("merchant_id", &merchant_id);
 
     let output = func(state, auth_out, payload).await.switch();