Support external alertmanager target

[lucklove]

What problem does this PR solve?

Close #1025

What is changed and how it works?

Add ExternalAlertmanagers field in PrometheusSpec, then we can define external alertmanagers in prometheus config:

monitoring_servers:
  - host: 172.16.5.140
    external_alertmanagers:
      - host: 172.16.5.134
      - host: 172.16.5.139
        web_port: 9094

Check List

Tests

• Manual test (add detailed scripts or steps below)

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

Release notes:

NONE

[codecov-io]

Codecov Report

Merging #1149 (fc3093d) into master (3d00875) will decrease coverage by 13.84%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master    #1149       +/-   ##
===========================================
- Coverage   52.33%   38.49%   -13.85%     
===========================================
  Files         284      222       -62     
  Lines       20278    16012     -4266     
===========================================
- Hits        10613     6164     -4449     
- Misses       8002     8788      +786     
+ Partials     1663     1060      -603     

Flag	Coverage Δ
cluster	38.49% <0.00%> (-4.54%)	⬇️
dm	?
integrate	38.49% <0.00%> (-7.86%)	⬇️
playground	?
tiup	?
unittest	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/cluster/spec/prometheus.go	64.07% <0.00%> (-16.05%)	⬇️
pkg/repository/store/store.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/queue/any_queue.go	0.00% <0.00%> (-83.34%)	⬇️
pkg/repository/utils/hash.go	0.00% <0.00%> (-81.82%)	⬇️
pkg/meta/err.go	0.00% <0.00%> (-81.25%)	⬇️
pkg/telemetry/meta.go	0.00% <0.00%> (-80.00%)	⬇️
pkg/telemetry/scrub.go	6.66% <0.00%> (-80.00%)	⬇️
pkg/cluster/api/error.go	0.00% <0.00%> (-80.00%)	⬇️
pkg/repository/store/local.go	0.00% <0.00%> (-76.48%)	⬇️
pkg/utils/semver.go	25.00% <0.00%> (-75.00%)	⬇️
... and 177 more

---

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[ti-chi-bot]

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[AstroProfundis]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: dee2467