Metrics: deprecate legacy metric types

In person discussions yielded the following proposal:

* Transition a model of only supporting ```Counter::Long``` and ```Gauge::Double``` metric types.
* Existing metrics that can not be represented as ```Counter::Long``` or ```Gauge::Double``` may provide value, but should not be a 'metric'.
* Case by case evaluate if there are more appropriate ways other then 'metrics' to expose some of those values. For example the last_failure_dt_tm for pipeline configs is of value, but just not as part of the metrics API.
* Adopt a general rule of thumb that metrics should be useful when comparing the values over a time period, and could be placed in a graph such that they may be reasoned about in a meaningful way.

This issue is to identify all the metrics in core and supported plugins that are not of type ```Counter::Long``` and ```Gauge::Double``` and if they provide value, and find a new home for them or remove them. Once all core Logstash metrics are moved or removed, we can officially deprecate (with a warning in the log) these legacy types, and remove them in the next major release.

See related: https://github.com/elastic/logstash/issues/7509