Cleanup existing metric code

[andsel]

Metric part of Logstash is half implemented in Ruby and half in Java, with some extentions classes that are just needed to invoke Ruby code from the Java side. Removing some Ruby classes that are just wrapped to Java code (Gauge and Counter) and reimplementing MetricStore and Collector as pure Java classes would remove this requirement simplifying the development of new features on the metric part.

• Remove agent field from metric collector #17893
• Remove ruby Counter and Gauge metric classes leveraging the existing Java implementation Removal of Ruby bridge classes for Gauge and Counter #17858
• [ ] move metric store to Java, draft [WIP on-hold] Reimplement metric store in pure java #17890
• [ ] move collector to Java