Add support for Sneakers

[mikker]

Continued from #599

Updating the implementation and spec a little bit. I don't feel we need to include version checking in the spec unless explicitly testing with different versions.

@redwrx has an open PR here (jondot/sneakers#414) to pass the worker itself to middleware. Until that is merged I don't think there's a way to get the worker name for the transaction names. Too bad, and we fall back on the queue name.

@estolfo, how do you feel about marking Sneakers support as experimental until a way to get the worker name is provided? That way we won't need to do the whole version dance around breaking changes when we change the names of all Sneakers transactions?

[estolfo]

I think there's no harm in making it experimental for now. But I would also write a comment on the Sneakers PR you referenced to let them know you're hoping that it'll be merged soon. That seemed to work for this jondot/sneakers#367

[codecov-io]

Codecov Report

Merging #676 into master will increase coverage by 31.04%.
The diff coverage is 92.3%.

@@             Coverage Diff             @@
##           master     #676       +/-   ##
===========================================
+ Coverage   64.11%   95.16%   +31.04%     
===========================================
  Files          99      103        +4     
  Lines        2943     3205      +262     
===========================================
+ Hits         1887     3050     +1163     
+ Misses       1056      155      -901

Impacted Files	Coverage Δ
lib/elastic_apm/config.rb	84.31% <ø> (+9.8%)	⬆️
lib/elastic_apm/spies/sneakers.rb	92.3% <92.3%> (ø)
lib/elastic_apm/spies/json.rb
lib/elastic_apm/sql/signature.rb	98.63% <0%> (ø)
lib/elastic_apm/sql/tokenizer.rb	99.24% <0%> (ø)
lib/elastic_apm/sql/tokens.rb	100% <0%> (ø)
lib/elastic_apm/sql_summarizer.rb	100% <0%> (ø)
lib/elastic_apm/metrics.rb	94.64% <0%> (+3.57%)	⬆️
lib/elastic_apm/metrics/cpu_mem_set.rb	97.95% <0%> (+4.08%)	⬆️
... and 74 more

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[estolfo]

There's a minor version mistake in the docs

[estolfo]

another minor comment