fix(dashboards): source Router Summary cluster variable from mongos metric (PMM-15174)

[theTibi]

Summary

• Bug: In the MongoDB Router Summary dashboard, the Cluster template variable shows wrong values (or nothing), leaving the router panels empty.
• Root cause: The cluster variable was populated from the mongod/replset metric mongodb_mongod_replset_my_state, while the dashboard's other cascading variables (service_name, node_name) and panel filters are all scoped to the mongos router metric mongodb_mongos_sharding_shards_total. The cluster label values on the mongod (shard) services do not necessarily match those on the mongos (router) services, so the dropdown was populated from the wrong metric.
• Fix: Source the cluster variable from mongodb_mongos_sharding_shards_total (both definition and query), matching service_name/node_name and listing exactly the clusters that have a mongos router — as requested in the ticket.

- label_values(mongodb_mongod_replset_my_state{environment=~"$environment"},cluster)
+ label_values(mongodb_mongos_sharding_shards_total{environment=~"$environment"},cluster)

Ticket

• PMM-15174

Test plan

• python3 dashboards/misc/cleanup-dash.py --check-only dashboards/dashboards/MongoDB/MongoDB_Router_Summary.json passes (no formatting changes)
• JSON is valid
• On a PMM Server monitoring a sharded MongoDB with mongos routers: the Cluster dropdown lists the sharded cluster(s) with routers, and selecting one populates service_name/node_name and renders data in the router panels
• Cluster list is consistent with the MongoDB Sharded Cluster Summary dashboard
• pmm-submodules (FB) PR: PMM-15174-fix-router-cluster-var (FB) Percona-Lab/pmm-submodules#4462

Notes

• Out of scope: the Routers panel also queries the mongod metric mongodb_mongod_replset_my_state; this PR intentionally changes only the cluster variable per the ticket. A follow-up can address the panel if needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.78%. Comparing base (31318c7) to head (56f866d).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5608      +/-   ##
==========================================
+ Coverage   43.59%   43.78%   +0.19%     
==========================================
  Files         415      545     +130     
  Lines       43134    45087    +1953     
  Branches        0      591     +591     
==========================================
+ Hits        18804    19742     +938     
- Misses      22454    23407     +953     
- Partials     1876     1938      +62     

Flag	Coverage Δ
admin	34.78% <ø> (ø)
unittests	41.29% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.