Rework flexible PublicMetrics registration introduced in 2be6b3e4 to
restore compatibility with v1.1 VanillaPublicMetrics. The new
MetricReaderPublicMetrics class now exposes metrics from a MetricReader
and VanillaPublicMetrics is deprecated. The MetricsEndpoint can now
exposes a collection of PublicMetric interface directly.
See gh-1094
14c6243637