Add before_emit callback for metrics

[sl0thentr0py]

part of #2246

[codecov]

Codecov Report

Merging #2258 (da14eaf) into neel/metrics/span-aggregator (0df891d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@                       Coverage Diff                        @@
##           neel/metrics/span-aggregator    #2258      +/-   ##
================================================================
+ Coverage                         97.54%   97.57%   +0.02%     
================================================================
  Files                               112      112              
  Lines                              4108     4116       +8     
================================================================
+ Hits                               4007     4016       +9     
+ Misses                              101      100       -1     

Components	Coverage Δ
sentry-ruby	98.28% <100.00%> (+0.03%)	⬆️
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/configuration.rb	98.79% <ø> (-0.02%)	⬇️
sentry-ruby/lib/sentry/metrics/aggregator.rb	99.07% <100.00%> (+0.02%)	⬆️
sentry-ruby/lib/sentry/metrics/configuration.rb	100.00% <100.00%> (ø)
...-ruby/lib/sentry/utils/argument_checking_helper.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[antonpirker]

Look good from my side