Implement sentry-trace propagation

[st0012]

This implements and closes #1445

Summary

The SDK will insert the sentry-trace to outgoing requests made with Net::HTTP. Its value would look like d827317d25d5420aa3aa97a0257db998-57757614642bdff5-1.

If the receiver service also uses Sentry and the SDK supports performance monitoring, its tracing event will be connected with the sender application's.

Example:

This feature is activated by default. But users can use the new config.propagate_traces config option to disable it.

[codecov-commenter]

Codecov Report

Merging #1446 (be751a3) into master (5ce32c1) will increase coverage by 0.62%.
The diff coverage is 100.00%.

❗ Current head be751a3 differs from pull request most recent head 4b68d22. Consider uploading reports for the commit 4b68d22 to get more accurate results

@@            Coverage Diff             @@
##           master    #1446      +/-   ##
==========================================
+ Coverage   98.22%   98.85%   +0.62%     
==========================================
  Files         213      118      -95     
  Lines       10034     6261    -3773     
==========================================
- Hits         9856     6189    -3667     
+ Misses        178       72     -106     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry-ruby.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/configuration.rb	97.95% <100.00%> (+0.02%)	⬆️
sentry-ruby/lib/sentry/hub.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/net/http.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/hub_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/net/http_spec.rb	100.00% <100.00%> (ø)
...raven/spec/raven/integrations/rack_timeout_spec.rb
...ations/rails/overrides/debug_exceptions_catcher.rb
sentry-raven/spec/raven/utils/request_id_spec.rb
...ven/breadcrumbs/active_support_breadcrumbs_spec.rb
... and 91 more

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[rhcarvalho]

I think we just need a method name change to match the SDK guidelines and we're good to merge.

Thanks @st0012 !

[st0012]

@rhcarvalho let's move the discussion here.
Another reason I choose Hub instead of Client is because in Hub we can also access the current scope.

Consider that this method needs to access different both Client and Scope, I still think Hub is currently the best place to have this method.

[rhcarvalho]

#1446 (comment)
I'd say we don't need the Hub because it is only used to get the current scope when the span is nil, but we never call it with a nil span.

I have the impression this line is unnecessary, isn't it? The only non-test place where this method is called is set_sentry_trace_header and we never call with a nil span.

So my thought was that this didn't necessarily need to access the current scope.

[st0012]

I thought being able to read the span from current scope is a hard requirement when I checked sentry-python's implementation.

https://github.com/getsentry/sentry-python/blob/4c09f3203d6d19789c6fa729a2e46557ad4ea913/sentry_sdk/hub.py#L692-L694

But if it's not, I'm happy to remove it and move this method to Client.

[rhcarvalho]

Thanks @st0012 !

[st0012]

@rhcarvalho thanks for the review!