Emit metrics / events for monitoring service brokers

Note: this has been re-opened following this comment: https://github.com/cloudfoundry/cloud_controller_ng/pull/1165#issuecomment-432995004

A short explanation of the proposed change

Emit extra http status code metrics for certain API endpoints, namely service broker-related endpoints. HTTP requests with the following prefixes will send extra metrics:

/[version]/service_instances
/[version]/service_bindings
/[version]/service_keys

The emitted metric name is:

cc.requests.#{route}.#{method.downcase}.http_status.#{status.to_s[0]}XX

E.g. cc.requests.service_bindings.put.http_status.5XX

An explanation of the use cases our change solves

Monitoring of service brokers to ensure operations are not failing, such as binding for example.

The current metrics only cover things internal to Cloud Foundry. These metrics provide the opportunity to monitor external components the platform relies on.

Links to any other associated PRs

Associated issue: https://github.com/cloudfoundry/cloud_controller_ng/issues/929
Resurrected from: https://github.com/cloudfoundry/cloud_controller_ng/pull/1165

• I have reviewed the contributing guide

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

• I have made this pull request to the master branch

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

• I have run CF Acceptance Tests

Start by reviewing the service broker API endpoint prefixes listed in the issue and the history of pull request 1165 and issue 929. Run bundle exec rake while working, and consider the work done when those endpoints emit metrics matching the specified route, method, and HTTP status format.