grafana: add max ping latency metric

[yibin87]

close: #1071

Summary

• keep the existing Ping Latency series for raw probe duration
• add a Max Ping Latency series using max_over_time on probe_duration_seconds over 1 minute for the selected PingJobHost
• align the new series with the BLACKER_ping_latency_more_than_1s alert logic while preserving PingJobHost filtering

Validation

• git diff --check
• reviewed the updated Grafana panel JSON diff

[pingcap-cla-assistant]

All committers have signed the CLA.

[yibin87]

/cc @csuzhangxc

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-04-07 06:39:20.436227629 +0000 UTC m=+851965.641587676: ☑️ agreed by csuzhangxc.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csuzhangxc

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• OWNERS [csuzhangxc]

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[yibin87]

/cherry-pick release-8.5

[ti-chi-bot]

@yibin87: new pull request created to branch release-8.5: #1072.

Details

In response to this:

/cherry-pick release-8.5

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