Report dropped spans in client reports

[sl0thentr0py]

• In addition to transactions, we want to also count all potential span drops individually in Client Reports
• This is helpful for collecting stats and for customer support for our new span based pricing model

closes #2341

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.66%. Comparing base (36866c5) to head (8ae78fb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2346      +/-   ##
==========================================
- Coverage   98.66%   98.66%   -0.01%     
==========================================
  Files         205      205              
  Lines       13483    13546      +63     
==========================================
+ Hits        13303    13365      +62     
- Misses        180      181       +1     

Components	Coverage Δ
sentry-ruby	99.02% <100.00%> (-0.01%)	⬇️
sentry-rails	97.41% <ø> (ø)
sentry-sidekiq	97.01% <ø> (ø)
sentry-resque	96.79% <ø> (ø)
sentry-delayed_job	98.92% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files	Coverage Δ
sentry-ruby/lib/sentry/client.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/envelope.rb	98.33% <100.00%> (ø)
sentry-ruby/lib/sentry/transaction.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/transport.rb	99.04% <100.00%> (ø)
...ntry-ruby/spec/sentry/client/event_sending_spec.rb	99.68% <100.00%> (+0.05%)	⬆️
sentry-ruby/spec/sentry/envelope_spec.rb	100.00% <ø> (ø)
sentry-ruby/spec/sentry/transaction_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/transport_spec.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[solnic]

Left a couple minor suggestions, otherwise LGTM!