Reduce cardinality of some of GitLab's Prometheus metrics and fix observed duration reporting.
Closes#41045
See merge request gitlab-org/gitlab-ce!15881
i.e.
Using compare and swap we update the expires_at value.
The thread that actually is able to update this value will also set
the cache holding method_call enabled state
Previously we'd create a separate Metric instance for every method call
that would exceed the method call threshold. This is problematic because
it doesn't provide us with information to accurately get the _total_
execution time of a particular method. For example, if the method
"Foo#bar" was called 4 times with a runtime of ~10 milliseconds we'd end
up with 4 different Metric instances. If we were to then get the
average/95th percentile/etc of the timings this would be roughly 10
milliseconds. However, the _actual_ total time spent in this method
would be around 40 milliseconds.
To solve this problem we now create a single Metric instance per method.
This Metric instance contains the _total_ real/CPU time and the call
count for every instrumented method.
