The advisory parameter explicit_bucket_boundaries allows instrumentation
authors to set bucket boundaries for histograms that differ from the
default bucket set. Since this only applies to histograms, advice
is not explicitly assigned in other instruments, however, shared methods
will have an empty hash assigned as the default argument.

Any key passed to advice will be accepted, but only the
:explicit_bucket_boundaries symbol will have an effect.

…nto metrics-advisory-parameters

The API doesn't have the functionality tested

must_equal(nil) will be removed in Minitest 6

…e/opentelemetry-ruby into metrics-advisory-parameters

Assign the advice instance variable in histograms only.
Advice will default to nil in all cases.

This reverts commit d2318b0.

This reverts commit ed4ceca.