[Java] Add support for Sentry Metrics about sentry-java HOT 3 CLOSED

Having looked into the docs and the .NET implementation I want to highlight a few development aspects:

• The product specs describe the overall API behavior best
• We not only need to provide APIs on the SDK side, but also need some basic logic to aggregate metrics into 10s buckets
• We need some async flush functionality to send past buckets to our backend
• There's a new envelope item, type = statsd for sending metrics
• We have to keep thread safety (metrics can be emitted by any thread, reading and cleaning up happens every 10s) in mind
• There's a timing API which connects spans with metrics AND creates gauge metrics aka "summaries" of metrics on a span level
• When capturing metrics, stack traces are used to determine a code location for an emitted metric, if feasible at all, some caching needs to be done

To get going fast let's split this up into smaller PRs / tasks:

• Implement basic API, add protocol definitions for sending statsd envelope items
• Add support for code locations, linking metrics to source code
• Add support for the timing API, connecting spans with metrics and creating gauges

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The product specs describe the overall API behavior best

@markushi do you mean the develop spec? the link is to develop docs, not product

Thanks for investigating in more detail!

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released with 7.6.0 (excluding code location, which is currently blocked and tracked separately)

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