metrics: grafana dashboards support multiple clusters

[zjj2wry]

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

[CLAassistant]

All committers have signed the CLA.

[sre-bot]

Please follow PR Title Format:

• pkg [, pkg2, pkg3]: what's changed

Or if the count of mainly changed packages are more than 3, use

• *: what's changed

[sre-bot]

No release note, Please follow https://github.com/pingcap/community/blob/master/contributors/release-note-checker.md

[zjj2wry]

/cc @DanielZhangQD

[zjj2wry]

/cc @tennix @breeswish @AstroProfundis

[jackysp]

/run-monitor-test

[sre-bot]

Visit the grafana server at: http://172.16.5.5:31657, it will last for 5 hours

[jackysp]

/approve

[jackysp]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 22482

[ti-srebot]

/run-all-tests

[ti-srebot]

@zjj2wry merge failed.

[breezewish]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 22508

[ti-srebot]

/run-all-tests

[zjj2wry]

/label needs-cherry-pick-4.0

[zjj2wry]

/run-cherry-picker

[ti-srebot]

cherry pick to release-4.0 in PR #22534