doc: Initial documentation of the lib/metrics

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/lib/metrics/lib_metrics.md

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+@dir /lib/metrics
+@brief lib/metrics: Metrics collection API
+
+This module is used for adding "metrics" support to Tor.
+
+Metrics are a collection of counters that are defined per-subsystem and
+accessed through the MetricsPort. Each subsystem is responsible for populating
+metrics store(s) and providing access to them through the `.get_metrics()`
+call located in the `subsys_fns_t` object.
+
+These metrics are meant to be extremely lightweight and thus can be accessed
+without too much CPU cost.