feat(spans): Add a metric to count spans

[phacops]

No description provided.

[codecov]

Codecov Report

Merging #59722 (e73f8f1) into master (c1930a7) will increase coverage by 0.00%.
Report is 16 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #59722   +/-   ##
=======================================
  Coverage   80.76%   80.76%           
=======================================
  Files        5173     5173           
  Lines      226854   226862    +8     
  Branches    38149    38149           
=======================================
+ Hits       183218   183233   +15     
+ Misses      38081    38076    -5     
+ Partials     5555     5553    -2     

Files	Coverage Δ
src/sentry/integrations/discord/client.py	100.00% <100.00%> (ø)
src/sentry/integrations/discord/commands.py	100.00% <100.00%> (ø)
src/sentry/integrations/discord/integration.py	100.00% <100.00%> (ø)
src/sentry/search/events/datasets/metrics.py	83.24% <ø> (ø)
src/sentry/search/events/datasets/metrics_layer.py	84.90% <ø> (ø)
src/sentry/sentry_metrics/indexer/strings.py	100.00% <ø> (ø)
static/app/components/forms/formPanel.tsx	100.00% <ø> (ø)
static/app/components/sidebar/index.tsx	91.66% <ø> (ø)

... and 9 files with indirect coverage changes