Log incomplete requests when stopping CC

[philippthun]

Thanks for contributing to cloud_controller_ng. To speed up the process of reviewing your pull request please provide us with:

• A short explanation of the proposed change:
  When CC receives the signal to stop execution, it simply aborts outstanding requests. With this change, all incomplete requests will be written into the log file when (after) the Thin server is stopped.

  The request logging code has been restructured similar to Metrics::RequestMetrics.

• An explanation of the use cases your change solves
  This change enables operators to identify requests that have not been completed due to a restart of Cloud Controller. Without this change it was required to search through the logs and compare the Started ... and Completed ... messages in order to identify which requests were still processed when CC stopped. This helps analyzing potentially blocking and/or slow requests that need to be improved.

• Links to any other associated PRs

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[elenasharma]

When reviewing this, might be handy to check out cloudfoundry/capi-release#195

[sweinstein22]

@sethboyles and I looked through the changes, agreed that they look reasonable, and ran through the following acceptance steps:

Acceptance:

• deployed changes to a boshlite
• set the apps index endpoint to sleep 1000, which is longer than the 30 second graceful shutdown timeout allowed per Invoke restart drain for failed healthcheck capi-release#195
• ran cf apps -v and saw a call to /v3/apps?order_by=name&space_guids=GUID stall out
• bosh ssh'd onto the api vm, ran monit stop cloud_controller_ng
• checked /var/vcap/sys/log/cloud_controller_ng/cloud_controller_ng.log and found instances of log lines mirroring those outlined in this change, Incomplete request: GET \"/v3/apps?order_by=name&space_guids=GUID

Thanks for the contribution!