[Option 1] Add clickhouse_node tag to query metrics

[sangeetashivaji]

What does this PR do?

Adds a clickhouse_node tag to ClickHouse DBM query metrics so per-node data can be sliced in-app. Query metrics previously read hostName() as server_node only for per-node checkpointing, then collapsed the node dimension via _merge_rows_across_nodes before emitting.

This change:

• Preserves the per-(query, node) dimension by carrying server_node on each row and removing the cross-node merge.
• Groups rows by node and submits one query-metrics payload per node, appending clickhouse_node:<node> to that payload's tags (reusing CLUSTER_NODE_TAG from utils.py, the same key the standard non-DBM metrics already emit).
• Includes server_node in the FQT row-identity key so the same query on different nodes is tracked distinctly. FQT events themselves are unchanged (node is not a meaningful dimension for the query-text catalog).

This is "Plan 1" (agent-only delivery). A follow-up "Plan 2" will align with the DBM-standard row-field + backend-materialization pattern used by postgres/mysql/sqlserver.

Motivation

The standard (non-DBM) ClickHouse metrics already tag per-node data with clickhouse_node, but the DBM checks do not expose that dimension. This brings the node-level tag to query metrics first, as the initial step of rolling it out across all DBM checks.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[dd-octo-sts]

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🎯 Code Coverage (details)
• Patch Coverage: 95.65%
• Overall Coverage: 94.00%

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