RED-199772: Refine metrics documentation

[peperon]

With this PR, we are refining the Metrics v2 documentation. It includes several improvements:

• Fixed issues and mismatches in the type column, including ensuring that the type column is also present for shard metrics.
• Added a new column for metric units, which reflects the exact type of each emitted metric.
• Corrected several metric names and descriptions, and removed duplicate metrics.
• Added redis_server_slave_buffer, a new metric that was recently introduced.

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Note

Low Risk
Documentation-only changes to an embedded markdown reference; no runtime code, but dashboard owners should verify queries against renamed cluster metrics and updated enum values.

Overview
Updates the shared rs-prometheus-metrics-v2.md embed used across Redis Software Prometheus docs.

All metric sections (database, node, cluster, replication, shard) now use a Unit column (bytes, count, microseconds, etc.) alongside corrected Type values—for example db_config is documented as a gauge (not a counter), latency histograms keep histogram type with explicit microsecond units, and shard rows gain consistent Type/Unit columns where they were missing before.

Cluster entries are aligned with emitted names/labels: label cluster_wd → node, total_node_count → total_nodes_count, and primary-election counters are consolidated to primary_selections_total (the separate “ended” counter row is removed). Replication docs fix database_syncer_current_status (out of sync 1, not 2) and spell out connectivity gauge meanings.

Shard docs add redis_server_slave_buffer, drop a duplicated block of RediSearch metric rows, and clarify several status gauges (e.g. redis_server_up, redis_server_master_link_status). db_status documents 99 = unknown.

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