Adds HoodieMetrics#emitTableVersionMetric(int) which registers a
"table.tableVersion" gauge in the metrics registry. BaseHoodieClient
calls it once per client construction after loading the table's
metaClient.

Motivation
----------
The existing metrics surface does not expose the on-disk Hudi table
version at all. Operators upgrading across major Hudi versions
(particularly anyone running with hoodie.write.auto.upgrade=false
and a pinned hoodie.write.table.version) currently have no
observability signal to alert when a table is unexpectedly promoted
to a newer table version by an upgrade path. This change fills the
gap with a single one-shot gauge that fires on every write /
table-service client init, so standard alerting
(Prometheus / Datadog / Graphite / JMX) can flag the drift in near
real time.

Behavior
--------
- Gauge name: <metric.prefix>.<tableName>.table.tableVersion
- Value: integer HoodieTableVersion#versionCode() (e.g. 6, 8)
- Emitted at the end of BaseHoodieClient construction, once.
- Best-effort: if the table has not yet been initialized
  (first-ever write to a new base path), the metaClient load throws
  and we log at DEBUG and skip; the gauge will surface on the next
  client init after the table exists.
- No-op when metrics are disabled (config.isMetricsOn() == false).

Test plan
---------
Added two unit tests to TestHoodieMetrics:
- testEmitTableVersionMetric: asserts the gauge is registered with
  the supplied version code.
- testEmitTableVersionMetricWhenMetricsDisabled: asserts no-op when
  metrics are off.

Compatibility
-------------
Pure additive change. No on-disk state, no schema change, no
behavior change for tables that disable metrics. Existing callers
unaffected.