Issue:
[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

Resolution:
<think>
PR Context:
- Title: refactor: set merchant_id as `MERCHANT_ID_NOT_FOUND` for traces and metrics if not found
- Merged: 2023-09-17T16:05:46Z
- Files changed: 1 files
- Commits: 3 commits

Implementation approach:
## 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
- [ ] Thi
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** refactor: set merchant_id as `MERCHANT_ID_NOT_FOUND` for traces and metrics if not found (#2156)
Files to modify:


**crates/router/src/services/api.rs**
  Modify existing code