chore: adding interceptor for getting headers before each request

[obecny]

Which problem is this PR solving?

• Fixes Support for dynamic headers in Zipkin exporter #2012

Short description of the changes

• Adds optional interceptor for getting headers before each request

[codecov]

Codecov Report

Merging #2050 (2f8e861) into main (4ef22ca) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 2f8e861 differs from pull request most recent head bb1ba37. Consider uploading reports for the commit bb1ba37 to get more accurate results

@@            Coverage Diff             @@
##             main    #2050      +/-   ##
==========================================
+ Coverage   92.73%   92.75%   +0.01%     
==========================================
  Files         137      138       +1     
  Lines        4915     4927      +12     
  Branches     1015     1017       +2     
==========================================
+ Hits         4558     4570      +12     
  Misses        357      357              

Impacted Files	Coverage Δ
...elemetry-exporter-zipkin/src/platform/node/util.ts	96.66% <ø> (ø)
...ackages/opentelemetry-exporter-zipkin/src/types.ts	100.00% <ø> (ø)
...ackages/opentelemetry-exporter-zipkin/src/utils.ts	100.00% <100.00%> (ø)
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	100.00% <100.00%> (ø)

[obecny]

@open-telemetry/javascript-approvers ^^, thx

[dyladan]

Seems to be a lot of function indirection here for quite a simple feature. Why do we have _beforeSend and _getHeaders and prepareGetHeaders and getHeadersBeforeSend?

Seems like the easiest solution would be more like:

class ZipkinExporter {
  // ...
  _getExportHeaders?: () => Map<string, string>;

  constructor(config: zipkinTypes.ExporterConfig) {
    // ...
    if (typeof config._getExportHeaders === 'function') {
      this._getExportHeaders = config._getExportHeaders
    }
  }
}

and in send you just do headers = this.__getExportHeaders?.() ?? {};

[dyladan]

Approving. Feel free to change the name or keep it