Metric summaries on span

[sl0thentr0py]

• add a LocalAggregator instance on spans to duplicate metrics on the span as a gauge metric
• proxy the main aggregator add calls to the local aggregator if a span is running
• start a metric.timing span in the Sentry::Metrics.timing API

part of #2246

[codecov]

Codecov Report

Merging #2255 (0df891d) into master (cf8f7ae) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 0df891d differs from pull request most recent head 9a7abaf. Consider uploading reports for the commit 9a7abaf to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2255      +/-   ##
==========================================
- Coverage   97.56%   97.54%   -0.03%     
==========================================
  Files         111      112       +1     
  Lines        4060     4108      +48     
==========================================
+ Hits         3961     4007      +46     
- Misses         99      101       +2     

Components	Coverage Δ
sentry-ruby	98.24% <100.00%> (-0.04%)	⬇️
sentry-rails	95.05% <ø> (ø)
sentry-sidekiq	94.70% <ø> (ø)
sentry-resque	90.76% <ø> (ø)
sentry-delayed_job	95.60% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files	Coverage Δ
sentry-ruby/lib/sentry/metrics.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/metrics/aggregator.rb	99.04% <100.00%> (-0.96%)	⬇️
sentry-ruby/lib/sentry/metrics/local_aggregator.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/span.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/transaction_event.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[antonpirker]

Looks like it does the same as Python. (Again, I am no Ruby dev so some things (in tests) look like black magic) But I guess this is ok.