[clickhouse] Skip emitting empty storage_health payloads

[sangeetashivaji]

Summary

Skip the storage_health event emission in ClickhousePartsAndMerges._emit_events when every collection (parts, merges, mutations, replication queue, detached parts, thresholds) is empty. Previously the integration emitted a payload every collection cycle regardless of whether the queries returned anything, including when every collector caught an exception and returned [].

Why this matters

The integration was sending one storage_health event per collection cycle (default 60s) for every
monitored ClickHouse instance, even when there was nothing to report. Two scenarios where this is
undesirable:

• Idle / empty instances: a fresh ClickHouse with no user tables produces no useful storage_health
  data, but the integration was still emitting an empty payload every minute.
• Total collection failure: when every collector hits an exception (auth issue, network blip,
  ClickHouse restart, restricted system tables on managed services), all eight collectors return [].
  The integration was emitting a payload claiming "everything is empty," which is misleading — we don't
  actually know the state of the database in that case.

The new guard makes the agent emit only when at least one collection has rows. For any production
instance with actual data, this is a no-op.

Behavior in real environments

The guard short-circuits only when every collection is empty:

Scenario	Result
Healthy production with data	parts and thresholds populated → emits as before
Healthy ClickHouse Cloud with data	parts populated → emits as before
Idle moment, busy database	parts/thresholds populated → emits as before
Fresh self-hosted, no user tables yet	thresholds populated → emits thresholds-only payload
Fresh ClickHouse Cloud, no user data, restricted system tables	all empty → skips
All collection queries fail (auth/network/restart)	all empty → skips

For any production customer with actual data, this is a no-op.

Test plan

• test_emit_events_shape — all collections populated, payload contains every section
• test_emit_events_uses_query_activity_channel_not_metadata — emits via the query-activity
  channel rather than the metadata channel (updated to use non-empty input)
• test_emit_events_skips_when_all_collections_empty — (new) every collection empty, no
  emission
• test_collect_and_emit_skips_when_all_collectors_empty — (new) end-to-end: every _collect_*
  returns empty, _collect_and_emit produces no emission
• test_collect_and_emit_runs_with_partial_failures — pre-existing, still passes: when only one
  collector returns rows, emission still happens

$ ddev --no-interactive test clickhouse -- -k "emit_events"
3 passed

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.11%. Comparing base (46de832) to head (b26b6cc).
⚠️ Report is 6 commits behind head on master.

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[datadog-official]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 92.91%

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[sangeetashivaji]

make every collector empty