chore(docs): document prometheus path label configuration options

[kalEl2001]

Document the collapse_request_paths_depth configuration option for Oathkeeper's
Prometheus metrics, introduced in ory/oathkeeper#.

The existing monitoring section only covered basic Prometheus setup (port, host,
metrics_path). This adds a "Controlling request path labels" subsection explaining
all three path label options (hide_request_paths, collapse_request_paths, and
the new collapse_request_paths_depth) with a comparison table and a config example.

Related Issue or Design Document

ory/oathkeeper#1212

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.com) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

[Copilot]

Pull request overview

Documents Oathkeeper’s Prometheus request-path label cardinality controls by adding a dedicated subsection to the Monitoring guide, including the newly introduced collapse_request_paths_depth option.

Changes:

• Add a “Controlling request path labels” subsection describing hide_request_paths, collapse_request_paths, and collapse_request_paths_depth.
• Include a comparison table and an example serve.prometheus configuration snippet.

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