feature: Metrics

[xrstf]

Feature Description

The agent should be able to emit OpenTelemetry metrics on a dedicated metrics address. I would like to see the amount of sync operations (not all reconciliations, but those were things actually change), optionally grouped by kcp workspace ID (this should be optional via CLI flag because of the high cardinality).

Proposed Solution

Integrate the necessary Go libraries and have at it.

Alternative Solutions

No response

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Additional Context

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