Why install Prometheus server?

> Please provide us with the following information:
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### This issue is for a: (mark with an `x`)
```
- [X] bug report -> please search issues before submitting
- [ ] feature request
- [X] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
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- Follow steps to in 'Install Prometheus' section and notice Prometheus server was installed with an external IP Address.
- Question, why install server when when it is not required by Azure Monitor for scraping?

### Any log messages given by the failure
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### Expected/desired behavior
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### OS and Version?
> Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

### Versions
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### Mention any other details that might be useful

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