Flexible registration of additional PublicMetrics

This commit permits the use of several PublicMetrics instances by
default. Previously, only one PublicMetrics service could be specified
and a user configuration would remove all the defaulting.

VanillaPublicMetrics now takes a collection of PublicMetrics and
invokes them in sequence to build the final collection of metrics.
The system-related metrics have been moved to SystemPublicMetrics and
are registered by default.

Also updated the documentation to mention this feature and how it
could be fully overridden.

Fixes gh-1094