feat: HTTP instrumentation: add the option to capture headers as span attributes

[seemk]

Which problem is this PR solving?

Adds the option to the HTTP instrumentation to capture headers as span attributes.
Relevant section in the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/semantic_conventions/http.md#http-request-and-response-headers

The headers which need capturing need to be set explicitly, by default no headers are transformed to span attributes.

For example if X-Forwarded-For is set to be captured on request headers, the span will have http.request.header.x_forwarded_for attribute set.

[codecov]

Codecov Report

Merging #2492 (d2c8c61) into main (33935e7) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2492      +/-   ##
==========================================
+ Coverage   93.20%   93.23%   +0.03%     
==========================================
  Files         137      137              
  Lines        5017     5043      +26     
  Branches     1060     1067       +7     
==========================================
+ Hits         4676     4702      +26     
  Misses        341      341              

Impacted Files	Coverage Δ
...es/opentelemetry-instrumentation-http/src/utils.ts	99.08% <0.00%> (+0.05%)	⬆️
...ges/opentelemetry-instrumentation-http/src/http.ts	94.90% <0.00%> (+0.25%)	⬆️

[Flarna]

Please update the readme with documentation of this new option.

[Flarna]

Suggested change

	response.setHeader('X-Server-Header1', 'server123');
	response.setHeader('X-ServEr-hEeader1', 'server123');

or similar to verify that casing doesn't matter

[seemk]

Thanks, added it!

[seemk]

Please update the readme with documentation of this new option.

Updated the README