Document transaction metrics

[axw]

We have added support in APM Server for:

• aggregating transaction duration metrics (Introduce transaction histogram metrics #3485)
• dropping unsampled transactions (Configuration for dropping non-sampled transactions #3702)

These can be controlled with the following config options:

• apm-server.sampling.keep_unsampled: true|false (currently defaults to true, for backwards compatibility)
  • Controls whether or not non-sampled transactions documents are recorded
• apm-server.aggregation.transactions.enabled: true|false (currently defaults to false)
  • Controls whether or not metrics are gathered and published
• apm-server.aggregation.transactions.interval: <duration> (currently defaults to 1m)
  • Controls how frequently the metrics are published
• apm-server.aggregation.transactions.max_groups: [1,+Inf] (currently defaults to 1000)
  • Maximum number of transaction groups to keep track of. Once this is exceeded, we devolve into recording a metrics document for each transaction not in one of the transaction groups being tracked.
• apm-server.aggregation.transactions.hdrhistogram_significant_figures: [1,5] (currently defaults to 2)
  • Transaction durations are aggregated using HDRHistogram. This option controls the number of significant figures to maintain for recorded values, which controls relative error. See also https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-percentile-aggregation.html#_hdr_histogram
• apm-server.aggregation.transactions.rum.user_agent.lru_size: [1,+Inf] (currently defaults to 5000)
  • For "page-load" RUM transactions we aggregate on the user-agent name, which requires user-agent parsing. To avoid parsing every user-agent, we maintain an cache of user-agent strings. This option controls the cache size.

There's a UI proof of concept underway in progress: elastic/kibana#66871 elastic/kibana#73953. I think we probably want to wait for the UI side to be in its shippable state before we document these options, otherwise they'll just be confusing.