Issue:
feat(router): adding metrics for tracking behavior throughout the `router` crate 

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates

## Description

This PR focuses on adding metrics to the `router` crate. 
> review-note: consider going through the `metrics.rs` file before reviewing the rest of the PR, while will provide more context while reviewing the rest of the code
<!-- Describe your changes in detail -->


### Additional Changes

- [ ] 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 its an obvious bug or documentation fix
that will have little conversation).
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## How did you test it?
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## 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 submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable


Resolution:
<think>
PR Context:
- Title: release: Update CHANGELOG for release v0.5.1
- Merged: 2023-03-24T06:55:06Z
- Files changed: 1 files
- Commits: 3 commits

Implementation approach:
# 0.5.1 (2023-03-21)

## Documentation Changes

* **try_local_system:**
  *  add Ubuntu on WSL2 setup instructions (#767) (1d2166cf)
  *  add API key creation step (#765) (4b268068)

## New Features

* **pm_list:**  handle client secret check (#759) (82344fc4)
*  add in-memory cache support for config table (#751) (abedaae4)
*  compile time optimization (#775) (5b5557b7)
* **router:**
  *  add support for stateful straight through routing (#752) (568bf01a)
  *  adding metrics for 
</think>

To implement this, you'll need to modify:


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