* allow exposing meta information for registered metrics

New public method `ExposeMetadata` allows enabling exposition
of dummy meta-info for all exposed metrics across all Sets.

This feature is needed to improve compatibility
with 3rd-party scrapers that require meta information to be present.

This commit doesn't update exposition of default system/process
metrics to keep the list of changes small. This change should
be added in a follow-up commit.

https://github.com/VictoriaMetrics/metrics/issues/48



* cleanup

* wip

* wip

* wip

* wip

---------

Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>