server: expose TSO batch size metric

## Bug report / enhancement request

The current PD Grafana panel for `TSO request batch size` depends on the client-side metric `pd_client_request_handle_tso_batch_size`. In stress tests with `pd-tso-bench`, this client metric can be missing or unrepresentative, making it difficult to validate real server-side batching behavior from PD monitoring.

## Expected behavior

1. PD server exposes a server-side TSO batch-size metric for direct observability.
2. Grafana TSO batch-size panel uses the PD server metric.

## Impact

- Hard to judge real TSO batch behavior from server perspective.

## Related PR

- https://github.com/tikv/pd/pull/10224