Allow plugins to register and report metrics

Current plugins have no way to register metrics(Corresponding MetricsRegistery utils code is in pinot-common), hence no metrics are really published along with each component.
We need to implement a way to dynamically extract metrics from plugins, so plugins can report metrics based on their own needs.
E.g. filesystem plugins should be able to report the API counter/latency metrics
Kafka plugins should be able to report topic partition information and consumer lag etc.

Start by examining the MetricsRegistery utility code in pinot-common and how current plugins are loaded and exposed by Pinot components. Define how plugins register metrics and how those metrics are dynamically extracted and published, using filesystem API counters/latencies and Kafka topic-partition and consumer-lag metrics as examples. Done means plugin-provided metrics are registered and reported by each component.

Written by the indexing model from the issue text.