docs: update monitoring documentation with accurate details

[krishnaGajabi]

Updates the monitoring docs for mayastor

[Copilot]

Pull request overview

This PR refreshes the Replicated PV Mayastor monitoring documentation by turning it into a consolidated “metrics reference” that covers both the I/O-engine Prometheus metrics exporter and the call-home stats exporter.

Changes:

• Replaces the previous exporter-focused guide with a structured metrics reference (disk pool, node, replica, volume).
• Adds guidance for deriving IOPS/latency/throughput from cumulative metrics, including reset handling.
• Documents call-home /stats lifecycle event counters and links to a preconfigured monitoring stack.

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[krishnaGajabi]

@tiagolobocastro should we change node_id label to nodein code just to be consistent with other metrics in the following newly added metrics

Metric	Type	Labels	Description
mayastor_node_cordoned	gauge	node_id	Indicates if the Mayastor node is cordoned
mayastor_node_draining	gauge	node_id	Indicates if the Mayastor node is draining
mayastor_node_online	gauge	node_id	Indicates if the Mayastor node is online

[tiagolobocastro]

@tiagolobocastro should we change node_id label to nodein code just to be consistent with other metrics in the following newly added metrics
Metric Type Labels Description
mayastor_node_cordoned gauge node_id Indicates if the Mayastor node is cordoned
mayastor_node_draining gauge node_id Indicates if the Mayastor node is draining
mayastor_node_online gauge node_id Indicates if the Mayastor node is online

hmm but changing the name may break existing user?

[krishnaGajabi]

@tiagolobocastro should we change node_id label to nodein code just to be consistent with other metrics in the following newly added metrics
Metric Type Labels Description
mayastor_node_cordoned gauge node_id Indicates if the Mayastor node is cordoned
mayastor_node_draining gauge node_id Indicates if the Mayastor node is draining
mayastor_node_online gauge node_id Indicates if the Mayastor node is online

hmm but changing the name may break existing user?

Yes (anyway users can for now can rename the label name with promql)