fix: update Prometheus source label (#3056)

[ti-chi-bot]

This is an automated cherry-pick of #3056

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What is changed, added, or deleted? (Required)

Update Prometheus source label from app_kubernetes_io_instance to pingcap_com_group

Since according to this, it uses the group name as the service name prefix.

Which TiDB Operator version(s) do your changes apply to? (Required)

• main (the latest development version for v2.x)
• v2.0 (TiDB Operator 2.0 versions)
• release-1.x (the latest development version for v1.x)
• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)

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[Oreoxmt]

/approve

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