Sentry Metrics for Java

[dingsdax]

https://develop.sentry.dev/sdk/telemetry/metrics/

Add support for Sentry Metrics to the Java SDK!

This will enable users to record counters, gauges, and distributions.

1. Define the Metrics Protocol and trace_metric envelope type in the SDK.
2. Add the Public API (SDK options and methods) as per the docs.
3. Make sure the the SDK follow the documented behavior.
4. Attach default attributes and optional user attributes to the SDK as per docs.
5. Follow requirements for rate limiting, buffering, batching & flushing and build in measures to prevent excessive memory usage.

For general feedback & questions about Sentry Metrics use this GH discussion; for Java SDK specific feedback & questions use the comment functionality on this GH issue.

[linear[bot]]

JAVA-240

[Angelodaniel]

Customer expressed the need for this as they are now forced to create Transactions to attach measurements to it which is not ideal.