metrics: grafana dashboards support multiple clusters (#22503)

[ti-srebot]

cherry-pick #22503 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/22534

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/22534:release-4.0-8520e763992e

---

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:
Many users(come from DBaaS、operator、tiup) monitor multiple TiDB clusters in one Grafana, this PR make Grafana dashboards support multiple clusters and not affect single cluster usage. Cluster variable is hidden by default.

What is changed and how it works?

What's Changed:

1. add a tidb_cluster label in all expr
2. add cluster variable in Grafana templating

How it Works:
load it to your Grafana :))

Single Cluster:
No change

Multiple Cluster:
Construct a label that can uniquely identify the cluster. in tidb_operator, you can use {namespace}-{cluster_name} as your {tidb_cluster} variable.
add this configuration to your prometheus config

relabel_configs:
  - source_labels:
      - namespace
      - name
    separator: "-"
    target_label: tidb_cluster

Related changes

• no related changes

Check List

Tests

• no code

Side effects

• no effects

Release note

• metrics: grafana dashboards support multiple clusters

[ti-srebot]

/run-all-tests

[ti-srebot]

@zjj2wry please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/tidb/invitations

[zjj2wry]

/run-all-tests

[zjj2wry]

/assign @zhongzc
PTAL

[zhongzc]

/lgtm

[zhongzc]

/merge

[ti-srebot]

Sorry @zhongzc, this branch cannot be merged without an approval of release maintainers.

[breezewish]

/merge

[ti-srebot]

Sorry @breeswish, this branch cannot be merged without an approval of release maintainers.

[bb7133]

/merge

[ti-srebot]

/run-all-tests

[bb7133]

/merge