Support transaction (part 1)

[st0012]

This PR builds the groundwork for the performance monitoring feature. It can already send the new "transaction" type data to the server now.

This feature can be enabled by setting:

Sentry.init do |config|
  # any float larger than 0.0 to activate the feature
  # but it doesn't do the sampling now and sends every transaction
  config.traces_sample_rate = 1.0
end

Every event happened inside a transaction will now have a new Trace Details section:

---

By clicking Search By Trace, users can see all the events associated with that transaction, including the transaction event itself.

---

And this is how a transaction event looks like:

Upcoming Features

• Sampling mechanism
• Different tracing events (currently only supports rack request and active record's SQL query)

[codecov-io]

Codecov Report

Merging #1108 (8fba220) into master (fa1216c) will increase coverage by 0.11%.
The diff coverage is 99.41%.

@@            Coverage Diff             @@
##           master    #1108      +/-   ##
==========================================
+ Coverage   97.92%   98.04%   +0.11%     
==========================================
  Files         133      144      +11     
  Lines        5601     6087     +486     
==========================================
+ Hits         5485     5968     +483     
- Misses        116      119       +3     

Impacted Files	Coverage Δ
...-ruby/spec/sentry/transport/http_transport_spec.rb	100.00% <ø> (ø)
sentry-ruby/spec/sentry/client_spec.rb	95.94% <94.11%> (-0.16%)	⬇️
...ls/lib/sentry/rails/tracing/abstract_subscriber.rb	94.44% <94.44%> (ø)
sentry-rails/spec/sentry/rails/tracing_spec.rb	97.29% <97.29%> (ø)
sentry-rails/lib/sentry/rails.rb	100.00% <100.00%> (ø)
sentry-rails/lib/sentry/rails/railtie.rb	85.00% <100.00%> (+1.21%)	⬆️
sentry-rails/lib/sentry/rails/tracing.rb	100.00% <100.00%> (ø)
...b/sentry/rails/tracing/active_record_subscriber.rb	100.00% <100.00%> (ø)
sentry-rails/spec/sentry/rails_spec.rb	100.00% <100.00%> (ø)
sentry-rails/spec/support/test_rails_app/app.rb	100.00% <100.00%> (ø)
... and 31 more

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[HazAT]

I think here is a nit, if sampled is nil which is a valid value we would emit #{@trace_id}-#{@span_id}- with a trailing -

we should change that (eventho I think it would still work)

[st0012]

if @samped is nil, sampled_flag will be an empty string, which will add nothing to the string and become

"#{@trace_id}-#{@span_id}-"

so exactly the expected behavior 😄

here's the spec for such case https://github.com/getsentry/sentry-ruby/pull/1108/files#diff-a54e868d59f29d1f5369002784782b73b90c18c6493aab761e0230a51329fb6bR52-R61