[Java] Add support for Sentry Metrics

[smeubank]

Description

Metrics! Sentry will support Metrics finally! First we need all of our SDKs to support Metrics, including Java and dependent SDKs

Requirements:

• Basic API Add metrics API #3205
• Metrics Summary Aggregate metrics for spans #3238
• Tag normalization Add metrics API #3205
• Before Emit Callback Add before_emit_metric callback to options #3250
• Span creating mode Add support for span creating mode in metrics.timing API #3248
• Proper changelog Unify Metrics changelog entry #3251
• Create docs for Java/Android/Platform Add metrics API docs for Java/Android sentry-docs#9374
• (Postponed for now) Code location~ [Metrics] Add code location support to Metrics #3241
  https://docs.sentry.io/product/metrics/metrics-set-up/

Question:

How much of this can the Android SDK inherit if added in Java?

All of it.

Document(s)

Develop Specs
SDK Develop Specs
Product Specs

[markushi]

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

[smeubank]

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!

[kahest]

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