fix(opentelemetry-exporter-prometheus)!: default exporter host to localhost

[cjihrig]

Which problem is this PR solving?

The Prometheus exporter's default host is undefined. The spec calls for the default to be 'localhost'.

Fixes #6594

Short description of the changes

This commit updates the Prometheus exporter's default host to be 'localhost' to match the spec. Tests were also updated as necessary to wait for the server to start.

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Existing test suite.

Checklist:

• Followed the style guidelines of this project

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.75%. Comparing base (8ee2a8b) to head (44071db).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6599      +/-   ##
==========================================
- Coverage   95.76%   95.75%   -0.02%     
==========================================
  Files         375      375              
  Lines       12739    12739              
  Branches     3013     3013              
==========================================
- Hits        12200    12198       -2     
- Misses        539      541       +2     

Files with missing lines	Coverage Δ
...etry-exporter-prometheus/src/PrometheusExporter.ts	96.70% <ø> (ø)

... and 1 file with indirect coverage changes

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[maryliag]

from @pichlermarc on otel-js-dev channel:
We are planning to publish SDK 3.0 around June/July, and have a milestone set up for it already. I'd suggest we do it there and communicate the change accordingly.

I'm not sure when we can start adding changes only for 3.0. Can you provide guidance @pichlermarc ?

[pichlermarc]

@maryliag - I thought it'd be best to do this at the same time as 3.x (possibly also bumping this package from experimental to stable in the process).

I think this change will break a lot of users that rely on the default behavior being that it binds to all available network interfaces instead of localhost.

suggestion: let's put a warning there for now, letting people know that this will only bind to localhost in the future. Once we're working on 3.x, we change the behavior and we release it together with 3.x, and include guidance on what to do in the migration document. This way people have some time to change their settings accordingly instead of being broken without warning.