feat: status code in gRPC request metric

[cv65kr]

Reason for This PR

roadrunner-server/roadrunner#1194

According to opened issue missing metric is Response codes

Description of Changes

Added response code tag in request_total

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[rustatian]

Thanks @cv65kr 👍🏻

[rustatian]

I updated the if-else statement to switch to use the advantage of Go's switch jump tables.