HTTPServerSettings: Support for additional headers

[ledor473]

Is your feature request related to a problem? Please describe.
When deploying an OpenTelemetry Collector with httpforwarder or healthcheckextension behind a caching proxy or a CDN, the GET requests gets cached and there's currently no options to pass additional headers on HTTPServerSettings.

Here are some links for various caching proxy that documents the headers that can be used to configure the cache behavior:

• https://www.varnish-software.com/developers/tutorials/http-caching-basics/
• https://docs.fastly.com/en/guides/caching-best-practices#cache-control-headers
• https://www.nginx.com/blog/nginx-caching-guide/

Describe the solution you'd like
Add a headers configuration to HTTPServerSettings.

It would be similar to the headers property on HTTPClientSettings:

opentelemetry-collector/config/confighttp/confighttp.go

Lines 57 to 60 in 687d5b7

	// Additional headers attached to each HTTP request sent by the client.
	// Existing header values are overwritten if collision happens.
	// Header values are opaque since they may be sensitive.
	Headers map[string]configopaque.String `mapstructure:"headers"`

Describe alternatives you've considered
I've looked at CORSSettings that is exposed but it only exposes MaxAge that ends up in Access-Control-Max-Age

[atoulme]

@ledor473 would you like to work on this issue? I'd be happy to assign it to you.

[ledor473]

If the proposal makes sense, I'd be happy to contribute it yes.

[atoulme]

not sure, you can bring it to the SIG meeting for discussion or open a PR to provoke thoughts.

[atoulme]

@ledor473 ready to close? This was shipped in 0.81.0.

[ledor473]

Yep! I thought it would auto-close. Closing..

[atoulme]

It closes only if in the PR you use the keyword Fixes, as in Fixes #foo