live timers: API boundary, implementation, opt-in (#14748)

* live timers: introduce API boundary

Introduces an API boundary for timers as a first-class metric, as described
in elastic/logstash#14675, and migrates all known internal timers to use the
new API boundary for tracked execution.

Please refer to the specification for details on motivations.

This commit is net zero change to behaviour, and introduces a single new
undocumented setting `metric.timers` to `logstash.yml`, which presently only
takes its default value `delayed` to indicate that delayed committing of
execution time is acceptable.

It implements the new `TimerMetric` API in a way that is also net-zero-change.
Tracked executions are still performed by marking a start time, performing
the tracked execution, and incrementing an underlying long-type counter with
the number of elapsed milliseconds _after_ execution has completed. This means
that long-running execution is still missing from the metric until it has
completed.

The new Timer API is available to both the Ruby- and the Java-based plugin APIs

* timer metrics: sub-package and add baseline tests

* WIP: move execution metric ownership out of queue

* noop: remove useless abstract method

Our `AbstractMetric` implements `Metric` and does not need to declare
an abstract override of `Metric#getType`. Doing so prevents interfaces
from providing a default override for all implementers.

* timer metric tests: extract util, refactor for reuse

* timers: accumulate milli-excess-nanos

* live timers: single-checkpoint implementation

* timer metric: use explicit type parameters to make intent clear

* remove unused imports

* use safe int conversion

* test fixup: use given name for tested metric

* test helper: TimerMetricFactory prefers nanotime supplier

* timers: flesh out test coverage, incl live-timers

* test: move validation of queue-read metrics to ObservedExecution

* flow: support non-moving denominator (±infinity)

* metrics: add metric config pass-through to env2yaml

docker/data/logstash/env2yaml/env2yaml.go

@@ -94,6 +94,8 @@ func normalizeSetting(setting string) (string, error) {
 		"log.level",
 		"log.format",
 		"modules",
+		"metric.collect",
+		"metric.timers",
 		"path.logs",
 		"path.plugins",
 		"api.auth.type",