[DBMON-6892] Fan out ClickHouse cluster queries over the resolved cluster name

[sangeetashivaji]

What does this PR do?

Fixes the clusterAllReplicas fan-out in single endpoint mode to use the cluster the instance actually belongs to, instead of a hardcoded 'default'.

• Adds ClickhouseCheck.fanout_cluster_name: the resolved cluster name, falling back to 'default' only when the deployment is not known to be self-hosted (Cloud always uses that name).
• get_queries() and get_system_table() fan out over that cluster, and read the local system.<table> when no cluster can be resolved.
• cluster_aware_query(base, cluster) now takes the cluster, and cluster names are quoted and escaped through quote_string() since they arrive as server-supplied data.

Split out of #24889, which now carries only the database_instance metadata additions and is stacked on top of this branch.

Motivation

Only ClickHouse Cloud names its cluster default. Against a self-hosted deployment a hardcoded 'default' either raises UNKNOWN_CLUSTER, or — when the stock localhost-only default cluster exists — silently returns the local node alone while the result is presented as a whole-cluster view.

Keeping this in its own PR keeps the changelog entries separate: this one is a fixed entry, #24889 is an added entry.

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• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
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[cit-pr-commenter-54b7da]

evalya-impact-summary

evalya impact analysis

Impact analysis: 0 selected, 0 skipped (of 0 test tasks)
Publish tasks:   1 (always emitted)
Diff (5 files):
  clickhouse/changelog.d/24920.fixed
  clickhouse/datadog_checks/clickhouse/clickhouse.py
  clickhouse/datadog_checks/clickhouse/utils.py
  clickhouse/tests/test_table_metrics.py
  clickhouse/tests/test_unit.py

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Require an unambiguous cluster before fanning out

When a node participates in multiple configured clusters and has no {cluster} macro, cluster_name comes from CLUSTER_NAME_QUERY, which deliberately uses ORDER BY cluster LIMIT 1 (utils.py:62-70); that makes the tag stable but does not establish that the selected topology matches the load-balanced endpoint. Returning that value here now routes the affected single-endpoint metrics and DBM queries through an arbitrary cluster, potentially omitting nodes or querying an unrelated topology. Use this value for fan-out only when resolution is unambiguous, or provide an explicit fan-out cluster setting.

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[sangeetashivaji]

Does make sense but its a feature functionality that needs to be built to support multi cluster setup for single endpoint mode in self-hosted which we do not want to do.

[sangeetashivaji]

all system tables in cloud are under default cluster, self hosted single_endpoint_mode is the only impacted path here

[dd-octo-sts]

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