Split out Instrumentation from Extractor

[chrisbutcher]

Introduces a new public method, GraphQLMetrics::Extractor#extract!, which takes in a GraphQL::Query and permits stateful extractors to be created that aren't used as instrumentation. See the test case below for an idea of its usage.

Only field_extracted, initialize and an attr_reader to pull out stored values is needed, but I implemented no-ops for the rest of the Extractor interface to ensure we detect breakage in those due to this or future changes.

[chrisbutcher]

Updated to extract a new boolean via variable_extracted called used_in_query which indicates if the variable was actually used in the selected operation.

Also introduces a running_as_instrumentation mode, to more cleanly branch between code paths that only make sense when running as instrumentation, vs. an ad hoc extractor.

[chrisbutcher]

tldr: We / other clients can use this to conditionally log, or log differently unused variables.

[chrisbutcher]

(Note to self: Wrap the clock_gettime calls within the gem within something like GraphQLMetrics::TimedBatchExecutor.current_time, so that we can stub that, instead of the stdlib method, with Process.stubs(:clock_gettime))

edit: Done.

[chrisbutcher]

@swalkinshaw @eapache At your earliest convenience, please see the latest 2 commits (perhaps start with the changes to README.md in the first commit).

This introduces a breaking change (the meaning of Extractor changed), so I'll be bumping the gem to 2.0 after this PR.

-class LoggingExtractor < GraphQLMetrics::Extractor
+class LoggingExtractor < GraphQLMetrics::Instrumentation

[chrisbutcher]

Thanks! Updated.

[swalkinshaw]

Awesome 🎉

[chrisbutcher]

@swalkinshaw Here's where I settled on the instrumentor/start/end time checks in extract_query.

start_time, end_time = if instrumentor
  start_time = instrumentor.ctx_namespace[Instrumentation::START_TIME_KEY]
  return unless start_time

  [start_time, Instrumentation.current_time]
end

[eapache]

I wonder if it would be cleaner to move logic like this into a method on the instrumentor, and then define a dummy instrumentor class to use instead of nil?

[chrisbutcher]

This is here since I'd rather default again at runtime if an extractor calls instrumentor, and the extractor overrode initialize but didn't call super.