docs: correct metric names in topics/metrics.md to match exported Prometheus names

[mlushpenko]

Fixes #1937

What's wrong today

docs/book/src/topics/metrics.md (rendered at https://secrets-store-csi-driver.sigs.k8s.io/topics/metrics) lists metric names that do not match what the driver actually exports on /metrics. Following open-telemetry/opentelemetry-go#3360, the OpenTelemetry Prometheus exporter appends _total to counter instruments, so the counter names on the wire end with _total rather than beginning with total_.

The e2e test at test/bats/e2e-provider.bats already asserts the real names:

assert_match "node_publish_total" "${output}"
assert_match "node_unpublish_total" "${output}"
assert_match "rotation_reconcile_total" "${output}"

…which contradicts the docs.

What this PR changes

Docs-only. Updates docs/book/src/topics/metrics.md:

Counter names — match what the Prometheus exporter emits:

Before	After
total_node_publish	node_publish_total
total_node_unpublish	node_unpublish_total
total_node_publish_error	node_publish_error_total
total_node_unpublish_error	node_unpublish_error_total
total_sync_k8s_secret	sync_k8s_secret_total
total_rotation_reconcile	rotation_reconcile_total
total_rotation_reconcile_error	rotation_reconcile_error_total

Histogram name — fixed typo; the instrument is registered as k8s_secret_duration_sec in pkg/secrets-store/stats_reporter.go, not sync_k8s_secret_duration_sec.

Tags on rotation_reconcile_total / rotation_reconcile_error_total — added the provider tag, which the reporter already sets at stats_reporter.go:146 and :155 but which was missing from the docs.

Sample output block — regenerated with the current names.

Two clarifications added:

1. A note explaining the OTEL _total suffix behavior (so future readers understand why the exported names differ from the instrument names registered in code).
2. A note that driver metrics only appear on pods that have actually served a mount / unmount / rotation reconcile — when port-forwarding a single pod for validation it's easy to hit an idle pod and see no driver metrics, which was confusing enough to motivate this issue.

No code changes. Metric names on the wire are unchanged.

Which issue this PR fixes

Fixes #1937

Special notes for your reviewer

None.

Does this PR introduce a user-facing change?

NONE

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[aramase]

/ok-to-test
/triage accepted

[aramase]

Thanks for the PR!

/lgtm
/approve

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 22.07%. Comparing base (fd0c075) to head (c379326).
⚠️ Report is 38 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2038   +/-   ##
=======================================
  Coverage   22.07%   22.07%           
=======================================
  Files          57       57           
  Lines        3198     3198           
=======================================
  Hits          706      706           
  Misses       2400     2400           
  Partials       92       92           

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[aramase]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aramase, mlushpenko

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[mlushpenko]

/retest