Initialize http_requests_total label values

What does this MR do?

Move http method/status initilaization from the http_requests_duration_seconds histogram to the http_requests_total counter. This is required to track error rates and error ratios.

For example, the ratio of 5xx requests to all requests.

sum(rate(http_requests_total{status=~"5.."}[5m])
/
sum(rate(http_requests_total[5m])

Without the inialization, there would be no values for 5xx on small or low traffic servers. This would lead to blank graphs.

gitlab-com/gl-infra/scalability#585

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team