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
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();

## 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

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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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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
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that will have little conversation).
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Fixes #2153 

## How did you test it?
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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
