[Enhancement] Expose the actual tablet version-limit counter (_approximate_num_rowsets) as a Prometheus metric

[jayakasadev]

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Description

Writes to a tablet are rejected once its version count passes max_tablet_version_num, and it fails with TOO_MANY_VERSION (E235). The check reads a counter called _approximate_num_rowsets.
The only related metric, tablet_version_num_distribution, reads a different counter that updates less frequently and skips tablets that are currently being written to. This hides potential issues until there is a failure.
There's no Prometheus metric for _approximate_num_rowsets. The only way to read it is via admin HTTP GET to /api/hotspot/tablet?metrics=num_rowsets.

Solution

1. Add a metric fed from _approximate_num_rowsets using the same pattern as the existing histogram in doris_metrics.{h,cpp}.
2. Report it for all tablets, including ones under active load

Are you willing to submit PR?

• Yes I am willing to submit a PR!

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