[receiver/prometheusreceiver] fix prom receiver config reload

[erikburt]

Description

Fixing a bug where reloading the Prometheus config, caused config entries of type Secret to be replaced with the literal value of <secret>.

• Switches to new yaml library which allows for custom marshaling (shoutout to @ridwanmsharif)
• Adds unit test to ensure reloading promreceiver config doesn't mask secrets

Link to tracking issue

Fixes #40520, #40916

Testing

• I've added a unit test, validating the Reload() method doesn't mask secrets for existing scrape_configs
• Pointed a local instance at a simple http server to ensure the credentials are not affected ([receiver/prometheusreceiver] basic authentication for urls stopped working #40520 (comment))
• Tested locally it with a target_allocator config, which was the original reason for the Reload() method, which introduced the bug (https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/40103/files#r2097520578)

Documentation

N/A

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: erikburt / name: Erik Burton (c8322a6, 4e06571, 9d4a0a0, b909a3a, a7da3a6, f70865f, 080c2c0, f45a115, 34c7e41, af48527, ab286db, 16381c9)

[ArthurSens]

LGTM, just some very small comments to make tests smaller. They are non-blocking of course!

I'd like to see another review from David or Krajo about the strategy as well, maybe they see something that I couldn't

[krajorama]

LGTM, I'm kicking myself for not finding this solution even though I thought of it and did some search on the current lib :( Nice job!

I've tested this locally with a simple target that uses basic auth.