Instrument serialization and rendering as 'format_response.grape'

[zvkemp]

A good portion of a grape API response is currently uninstrumented — when endpoint_run.grape closes, the response bodies still haven't been serialized or rendered into their appropriate templates. Adding format_response.grape instrumentation will allow us to measure time spent in this portion of the request.

[dblock]

LGTM, care to check out the build failure?

[zvkemp]

@dblock Failing on edge rails. Looks like it's related to the recent arel/activerecord merge.

Bundler could not find compatible versions for gem "activerecord":
  In rails_edge.gemfile:
    arel was resolved to 10.0.0.pre, which depends on
      activerecord (>= 6.a)

Unrelated to this PR, but I'd be happy to take care of it (could just tack on another commit to this one, unless you prefer to keep the issues separate).