RED-188637: Exposing DB tags to metrics

[petar-valchev-redis]

Context: https://redislabs.atlassian.net/browse/RED-188637

Documents how to expose Redis Software database tags as labels in v2 metrics via the db_tags metric.

• Add a Database tags in metrics page: enabling exposure, the db_tags metric and emission rules, join/query examples for Prometheus/Grafana, Datadog, New Relic, and Dynatrace, plus cardinality, security, and FAQ.
• Add a Metrics configuration page and the /v1/metrics_config REST reference and rladmin metrics command.
• Update the database tags page with validation rules, backward compatibility, and system tags.
• Add info metrics to rladmin info and the db_tags row to the v2 metrics reference.

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Note

Low Risk
Documentation-only changes with no runtime or API behavior modifications in this diff.

Overview
Documents how Redis Software database tags can be exported into v2 Prometheus metrics via the db_tags information metric, and how operators configure and consume that data.

Adds Database tags in metrics (enable expose_db_tags / metrics_tag_keys_exposed, emission rules, PromQL joins, and platform-specific enrichment for Prometheus/Grafana, Datadog, New Relic, and Dynatrace) plus Metrics configuration and the /v1/metrics_config REST object and requests. rladmin info metrics and rladmin metrics config are documented for the same cluster-wide settings.

The database tags page gains validation rules (Prometheus-compatible keys, limits, legacy unchanged tags), system tags, a metrics overview, and troubleshooting. The v2 metrics reference embed adds a db_tags row linking to the new guide.

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