Add Sentry.with_child_span for easier span recording

[st0012]

This PR adds a new Sentry.with_child_span API. It'll allow users to record a child span with a block without the need to getting the current span from scope, checking its presence...etc. A simple block wrapping will do the trick:

Sentry.with_child_span(op: "my op") do |child_span|
  # my operation
  # operation result will be the return value
end

Note that if there's no span in the current scope, the block will still be executed. But the yield child_span will be nil instead.

With this API, it's also easier to fix the child span nesting issue reported in #1723.

[codecov-commenter]

Codecov Report

Merging #1783 (4af72c0) into master (fa46e86) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1783      +/-   ##
==========================================
- Coverage   98.40%   98.40%   -0.01%     
==========================================
  Files         145      145              
  Lines        8480     8536      +56     
==========================================
+ Hits         8345     8400      +55     
- Misses        135      136       +1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry-ruby.rb	96.22% <100.00%> (+0.25%)	⬆️
...y-ruby/spec/sentry/rack/capture_exceptions_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry_spec.rb	99.76% <100.00%> (+0.01%)	⬆️
sentry-resque/lib/sentry/resque.rb	97.14% <0.00%> (-2.86%)	⬇️

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[st0012]

ping @sl0thentr0py

[sl0thentr0py]

Just an observation, all the current usages of set_span set it to the transaction and we don't really use scope#get_span either so this will be the first time the span on the scope gets used right?

➜  sentry-ruby git:(master) ✗ ag --ignore 'spec' set_span
sentry-rails/lib/sentry/rails/action_cable.rb
18:              scope.set_span(transaction) if transaction

sentry-rails/lib/sentry/rails/active_job.rb
31:                scope.set_span(transaction) if transaction

sentry-resque/lib/sentry/resque.rb
28:              scope.set_span(transaction) if transaction

sentry-delayed_job/lib/sentry/delayed_job/plugin.rb
22:            scope.set_span(transaction) if transaction

sentry-sidekiq/lib/sentry/sidekiq/sentry_context_middleware.rb
21:        scope.set_span(transaction) if transaction

sentry-ruby/lib/sentry/scope.rb
132:    def set_span(span)

sentry-ruby/lib/sentry/rack/capture_exceptions.rb
23:            scope.set_span(transaction) if transaction

[st0012]

Yes, because most the usages in the SDK are to create the top-level transaction. But as described in #1784, sentry-rails's subscriber should use spans instead. And users will generally use get_span and set_span for spans more than transactions.

[sl0thentr0py]

sorry for the delay, busy week :/

can you maybe add an integration test too where you add a child span with this new api in an existing request/response cycle transaction?

[st0012]

@sl0thentr0py I've added the integration test here.

[sl0thentr0py]

thx!