Add metric request error in log

[sylr]

Summary

Add the error in the logs

Related issues

N/A

User Explanation

I have those errors:

{"level":"error","config_file_source":"/etc/pomerium/config.yaml","bootstrap":true,"time":"2023-09-21T06:19:49Z","message":"responding to metrics request"}
{"level":"error","config_file_source":"/etc/pomerium/config.yaml","bootstrap":true,"time":"2023-09-21T06:20:04Z","message":"responding to metrics request"}
{"level":"error","config_file_source":"/etc/pomerium/config.yaml","bootstrap":true,"time":"2023-09-21T06:20:19Z","message":"responding to metrics request"}

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[coveralls]

coverage: 63.783% (+0.03%) from 63.752% when pulling 5ed8113 on sylr:show-metric-error into f313ff1 on pomerium:main.