[Milestone] Metrics Meta Data

[jan-auer]

DDM wants to show code locations with metrics. This is semi-static information, in that it changes infrequently between deploys, but doesn't need to be reported with every data point. The code locations should be shown in the DDM explorer.

Feature Set:

• Code Location
• (TBD) Cached span flag
• Descriptions

PoC

Goals:

1. Estimate cost scaling
2. SDK protocol + impl in Py
3. Transient storage
4. Expose in products

Constraints:

• Multiple locations per metric possible
• Concept of time (esp for endpoint)
• No explicit release-support, no location per tag
• No symbolication yet, but consider it for future
• No data scrubbing yet, but consider it for future
• Debouncing doesn't have to be persistent (ie. not survive restarts)

Design Doc

### Stage 1: PoC
- [x] Envelope Item + protocol
- [x] Define what is stored for locations
- [ ] https://github.com/getsentry/sentry-python/pull/2526
- [ ] https://github.com/getsentry/relay/pull/2751
- [ ] https://github.com/getsentry/sentry/pull/60461
- [ ] https://github.com/getsentry/getsentry/pull/12126
- [ ] https://github.com/getsentry/sentry/pull/60369
- [ ] https://github.com/getsentry/sentry/pull/60514
- [ ] https://github.com/getsentry/sentry/pull/60539
- [ ] https://github.com/getsentry/sentry/issues/60580
- [ ] https://github.com/getsentry/sentry/pull/60629

### Stage 2: Productionize for LA
- [ ] https://github.com/getsentry/sentry/issues/60684
- [ ] https://github.com/getsentry/relay/pull/2781
- [ ] https://github.com/getsentry/sentry-python/pull/2539
- [ ] https://github.com/getsentry/relay/pull/2784
- [ ] https://github.com/getsentry/relay/issues/2787
- [ ] Add frequency score to endpoint response
- [ ] https://github.com/getsentry/team-ingest/issues/255
- [ ] https://github.com/getsentry/relay/issues/2803
- [ ] https://github.com/getsentry/sentry-python/issues/2550

### Q1: GA Follow-up
- [ ] Permanent storage
- [ ] Re-Think batching per metadata
- [ ] JS sourcemap support for
- [ ] Native Symbolication