[Metrics] Add code location support to Metrics

[markushi]

Description

Feature description: https://develop.sentry.dev/sdk/metrics/#meta-data

As of now, languages which require symbolication are not supported.
The feature itself is still in a MVP/UX research phase and thus not connected to any processing pipelines. Once we have some more positive feedback on code locations, the backend will include code locations into event processing, adding the possibility to symbolicate those events / enrich them with source context.

Related Issue: #3190

[markushi]

FYI @kahest @smeubank after talking to @iambriccardo, this feature is blocked for now - which is fine, as it's optional anyway.