Inconsistent Metric Naming Convention

I think there are at least two different naming conventions followed for metrics.

One of them looks like:

```
pinot.server...
pinot.server..
```
(e.g. `realtimeIngestionDelayMs`, `segmentCount`)

And the other type looks like:

```
pinot.server..
```
(e.g. `deletedSegmentCount`)

The hierarchy between metric and table names is different. Is this intentional?

The issue names no files, tests, or entry points. Review the metric definitions and naming conventions across Pinot, determine whether the hierarchy difference is intentional, and document the expected convention or the scope of any required consistency change.

Written by the indexing model from the issue text.