Grape support

[estolfo]

This PR adds support for Grape. The changes are:

• Defining a Grape.start method so that the agent can be hooked up to a Grape app
• Normalizer for endpoint_run
• Set config framework values for Grape in Grape.start
• Documentation for Grape integration
• Add framework name and version to transaction context when a grape event is initiated by a Rails app.

Further work:

• Maybe add normalizer format_response
• Consider not instrumenting the filter and validator endpoints, as they can be very noisy. There are a lot of internal filters and validators called and there's no way to distinguish between library and user-specified ones.

Todo:

• Test in Jenkins

Closes #206, #321

[mikker]

Code looks good!

We also need to add the public api (and the Rails.start one too) to docs/api.asciidoc btw. And a changelog entry.

[estolfo]

@mikker btw, we already have documentation for Rails.start here

[codecov-io]

Codecov Report

Merging #562 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
- Coverage   94.95%   94.84%   -0.11%     
==========================================
  Files          87       87              
  Lines        2695     2696       +1     
==========================================
- Hits         2559     2557       -2     
- Misses        136      139       +3

Impacted Files	Coverage Δ
lib/elastic_apm/middleware.rb	97.22% <100%> (ø)	⬆️
lib/elastic_apm.rb	96.15% <100%> (+0.04%)	⬆️
lib/elastic_apm/config.rb	90.27% <0%> (-2.09%)	⬇️

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[mikker]

Could Arguments to .start(…) be explanatory enough? I'm afraid mentioning too many different frameworks here will confuse more than help?

[estolfo]

@mikker Since your approval, I've made significant changes to this PR. It'd be great if you could review the following:

• The new Rails and Grape integration test. It verifies that grape requests change the transaction context. They add the grape framework attributes even though the events are technically initiated by Rails.
• The ability to install and require multiple frameworks for testing so that rails and grape can be tested together in Jenkins.
• Checking that there is no current transaction before creating one in ElasticAPM::Middleware
• Framework attributes are added to the transaction context in the Grape endpoint event if the framework is set to Rails.

[mikker]

This is good to go 👍