[fix] Empty label is not allowed in Prometheus

[borissavelev]

For DM deployment, there are no blackbox instances, and the label value is replaced with an empty string.

New Prometheus versions have a check for that:

FAILED: instance 0 in group 0: no address
, ssh_stdout: Checking /tidb-deploy/prometheus-9090/conf/prometheus.yml , ssh_command: export LANG=C; PATH=$PATH:/bin:/sbin:/usr/bin:/usr/sbin /tidb-deploy/prometheus-9090/bin/prometheus/promtool check config /tidb-deploy/prometheus-9090/conf/prometheus.yml}, cause: exit status 1: check config failed

[CLAassistant]

All committers have signed the CLA.

[ti-chi-bot]

Welcome @borissavelev! It looks like this is your first PR to pingcap/tiup 🎉

[kaaaaaaang]

/lgtm

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (77e0b53) 55.42% compared to head (dabeb25) 53.36%.

❗ Current head dabeb25 differs from pull request most recent head 1d63294. Consider uploading reports for the commit 1d63294 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2349      +/-   ##
==========================================
- Coverage   55.42%   53.36%   -2.06%     
==========================================
  Files         329      329              
  Lines       34827    34827              
==========================================
- Hits        19300    18582     -718     
- Misses      13217    13983     +766     
+ Partials     2310     2262      -48     

Flag	Coverage Δ
cluster	41.93% <ø> (-2.79%)	⬇️
dm	25.45% <ø> (-0.05%)	⬇️
tiup	33.60% <ø> (ø)
unittest	22.34% <ø> (ø)

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[kaaaaaaang]

/approve

[ti-chi-bot]

@borissavelev: Your PR was out of date, I have automatically updated it for you.

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[kaaaaaaang]

/lgtm

[kaaaaaaang]

/approve

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-09 12:54:18.87648837 +0000 UTC m=+361448.460742043: ☑️ agreed by kaaaaaaang.
• 2024-01-11 08:31:19.930498595 +0000 UTC m=+518469.514752265: ✖️🔁 reset by ti-chi-bot[bot].
• 2024-01-11 08:50:18.548072381 +0000 UTC m=+519608.132326053: ☑️ agreed by kaaaaaaang.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kaaaaaaang

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