fix: headers are appended to existing one (open-telemetry#2335)

[niko-achilles]

Which problem is this PR solving?

• fixes: bug(exporter-collector): headers are appended to existing one #2335
• Needs Review and @vmarchaud may halp me given we were involved in the issue above ... :-)

Short description of the changes

• added file util.test.ts under packages/opentelemetry-exporter-collector/test/browser/util.test.ts
  which exercises the sendWithXhr method of packages/opentelemetry-exporter-collector/src/platform/browser/util.ts
• File packages/opentelemetry-exporter-collector/src/platform/browser/util.ts is changed to verify if the developer who uses the CollectorTraceExporter api has set explicit Content-Type and Accept Headers . If that is not the case , only then the sendWithXhrt method applies the Content-Type and Accept Headers with the value application/json.

[codecov]

Codecov Report

Merging #2357 (e7f00df) into main (c55142f) will increase coverage by 0.30%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2357      +/-   ##
==========================================
+ Coverage   92.47%   92.77%   +0.30%     
==========================================
  Files         122      145      +23     
  Lines        4132     5221    +1089     
  Branches      859     1070     +211     
==========================================
+ Hits         3821     4844    +1023     
- Misses        311      377      +66     

Impacted Files	Coverage Δ
...kages/opentelemetry-api-metrics/src/api/metrics.ts	90.90% <0.00%> (ø)
...-instrumentation-fetch/src/enums/AttributeNames.ts	100.00% <0.00%> (ø)
...ntelemetry-web/src/enums/PerformanceTimingNames.ts	100.00% <0.00%> (ø)
packages/opentelemetry-web/src/utils.ts	94.90% <0.00%> (ø)
...ry-context-zone-peer-dep/src/ZoneContextManager.ts	85.18% <0.00%> (ø)
...lemetry-exporter-collector/src/transformMetrics.ts	95.71% <0.00%> (ø)
...opentelemetry-api-metrics/src/NoopMeterProvider.ts	100.00% <0.00%> (ø)
packages/opentelemetry-web/src/types.ts	100.00% <0.00%> (ø)
...es/opentelemetry-context-zone-peer-dep/src/util.ts	100.00% <0.00%> (ø)
...s/opentelemetry-instrumentation-fetch/src/fetch.ts	96.91% <0.00%> (ø)
... and 13 more

[obecny]

just one last thing, other than that lgtm

[vmarchaud]

lgtm

[obecny]

nice and neat :), lgtm thx for changes

[dyladan]

Once the lint is fixed this is mergeable

[dyladan]

think there might be a github UI bug...

[dyladan]

Approving again to try to fix github