Tracing without performance

[sl0thentr0py]

This PR is 1/2 to enable Tracing without Performance, i.e. make sure all our events are connected even if they are not Transactions.

This enables use cases such as Errors / Transactions / Replays etc all being connected across services and not just Transactions.

Summary of changes

• new PropagationContext class that generates trace/span ids and baggage irrespective of whether there are transactions/spans active or not
• this lives on the Scope
• three new top level methods that first check the span and fallback on the scope's propagation context
  • Sentry.get_traceparent
  • Sentry.get_baggage
  • Sentry.get_trace_propagation_headers
• move dynamic_sampling_context to Event from TransactionEvent since all events will now have this info
• use the new top level helpers in net/http patch to set propagation headers

closes #2056
also see #2089

[codecov]

Codecov Report

Patch coverage is 64.46% of modified lines.

Files Changed	Coverage
sentry-ruby/lib/sentry/client.rb	ø
sentry-ruby/lib/sentry/rack/capture_exceptions.rb	0.00%
sentry-ruby/lib/sentry/transaction_event.rb	ø
sentry-ruby/lib/sentry/hub.rb	21.05%
sentry-ruby/lib/sentry/net/http.rb	28.57%
sentry-ruby/lib/sentry-ruby.rb	33.33%
sentry-ruby/lib/sentry/propagation_context.rb	77.35%
sentry-ruby/lib/sentry/scope.rb	77.77%
sentry-rails/lib/sentry/rails/action_cable.rb	100.00%
...entry-rails/lib/sentry/rails/capture_exceptions.rb	100.00%
... and 5 more

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[sl0thentr0py]

@st0012 still gonna write some specs and test but feel free to start reviewing if you have time and let me know if you have questions on why we are doing this!