SOLR-8785: Use Metrics library for core metrics

[randomstatistic]

There were three main areas that used the copied classes in org.apache.solr.util.stats:

• AnalyticsStatisticsCollector
• Overseer.Stats
• RequestHandlerBase

This patch adds depreciation tags to all the copied classes, and also replaces all usage of those classes with classes from the Metrics library.
I added one new class (org.apache.solr.util.stats.Metrics) to provide some common access patterns for metrics gathering.

This patch only adds Registry-based tracking to RequestHandlerBase, although all three areas are a fit for it. The effect is that all one needs to do is add a Reporter to the SharedMetricRegistry named “solr.registry.requesthandler” and all named request handler stats will be exported automatically.

Compatibility notes:

• The “totalTime” stat has been deleted from all three areas. This never seemed very useful, and Metrics didn’t support it in the Timer class, so it would have required some extra code to keep.
• RequestHandler stats are now persistent, and will no longer reset on reload.

[ctargett]

https://issues.apache.org/jira/browse/SOLR-8785 was implemented in Solr 6.4, so this PR can be closed.