metrics: support multiple k8s and tidb in grafana dashboards

[just1900]

Signed-off-by: just1900 legendj228@gmail.com

What problem does this PR solve?

Issue Number: close #1134

What is changed and how it works?

What's Changed:

• add a tidb_cluster label in all expr
• add a k8s_cluster label in all expr
• add tidb_cluster and k8s_cluster variable in Grafana templating

How it works:

• For users with single-k8s, nothing need to be changed, just use it as before.
• For users with multi-k8s:
  • first add kubernetes label to identify k8s cluster in your prometheus' external_labels.
  • set field "hide" to '0' to show variables in grafana via sed -i s/"hide": 2/"hide": 0/g <path/to/dashboard.json>
  • load it to grafana

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the tidb-ansible repository
• Need to be included in the release note

Release note

• support multiple k8s and tidb in grafana dashboards

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