Add doc for explaining the GC timesliced metrics in Java Agent

[tanaka-takayoshi]

How can we make our docs better?

A customer is confused about what is the definition of GC timesliced metrics such as the followings.

• 'GC/G1 Young Generation'
• 'GC/G1 Old Generation'
• 'GC/G1 Concurrent GC'

These metrics are queries by the following NRQL.

SELECT sum(newrelic.timeslice.value) FROM Metric WHERE metricTimesliceName LIKE 'GC%' AND appName = '<app-name>' FACET host, metricTimesliceName TIMESERIES

https://docs.newrelic.com/docs/apm/agents/java-agent/features/jvms-page-java-view-app-server-metrics-jmx/
says just Garbage collection: The garbage collection CPU time.

After reading the Agent code and JavaDoc, we can add the following explanation to the doc.

• GC metrics are based on the JVM MXBean metrics.
• While the JMX metrics are accumulated values, New Relic stores the values are difference between data points.
• You can query the number of collections that have occurred in specified time window with count(newrelic.timeslice.value) .
• You can query the collection elapsed time in milliseconds in specified time window with sum(newrelic.timeslice.value).

[github-actions]

Hi @tanaka-takayoshi 👋

Thank you for filing an issue! We'll triage your issue and let you know if we have questions, and then route it to the appropriate team so we can get it solved.