ref(spans): Reduce number of tags on span metrics (#2907)

During the development of Starfish we've repeatedly added new tags to
span metrics without restricting the conditions under which a tag is set
(that is, what queries actually require a tag).

This PR introduces a new config for the `exclusive_time` and
`exclusive_time_light` metrics, which limits the number of cases in
which the metrics and / or their tags are extracted. Some tags are
removed entirely.

For now, the new config will only be enabled for projects that have the
`all-modules` feature flag enabled, i.e. sentry-internal. After we've
verified that the new config covers all required cases, we can enable
the new config for all orgs.

ref: getsentry/team-ingest#252

CHANGELOG.md

@@ -12,6 +12,7 @@
 - Add `duration` metric for mobile app start spans. ([#2906](https://github.com/getsentry/relay/pull/2906))
 - Introduce the configuration option `http.global_metrics`. When enabled, Relay submits metric buckets not through regular project-scoped Envelopes, but instead through the global endpoint. When this Relay serves a high number of projects, this can reduce the overall request volume. ([#2902](https://github.com/getsentry/relay/pull/2902))
 - Record the size of global metrics requests in statsd as `upstream.metrics.body_size`. ([#2908](https://github.com/getsentry/relay/pull/2908))
+- Only extract span metrics / tags when they are needed. ([#2907](https://github.com/getsentry/relay/pull/2907))
 
 ## 23.12.1