Added ActivityEvent.EnumerateTags & ActivityLink.EnumerateTags extensions

[CodeBlanch]

Added extension methods for enumerating tags on Activity Events & Links without allocation. Updated Jaeger. Removed some code from the EnumerationHelper that is no longer needed. Renamed Activity.EnumerateTagValues -> Activity.EnumerateTags.

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
EnumerateNonemptyActivityLinkTags	68.63 ns	1.380 ns	2.022 ns	0.0057	-	-	48 B
EnumerateLinksNonemptyActivityLinkTags	46.23 ns	0.798 ns	0.747 ns	-	-	-	-
EnumerateNonemptyActivityEventTags	67.04 ns	1.344 ns	1.320 ns	0.0057	-	-	48 B
EnumerateLinksNonemptyActivityEventTags	46.22 ns	0.887 ns	1.122 ns	-	-	-	-

TODOs:

• CHANGELOG.md updated for non-trivial changes
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1320 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1320      +/-   ##
==========================================
+ Coverage   83.81%   83.88%   +0.07%     
==========================================
  Files         347      347              
  Lines       13362    13412      +50     
==========================================
+ Hits        11199    11251      +52     
+ Misses       2163     2161       -2     

Impacted Files	Coverage Δ
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...et/src/OpenTelemetry/Internal/EnumerationHelper.cs	87.09% <0.00%> (-1.48%)	⬇️
....Jaeger/Implementation/JaegerActivityExtensions.cs	96.59% <0.00%> (-0.12%)	⬇️
...penTelemetry.Tests/Trace/ActivityExtensionsTest.cs	100.00% <0.00%> (ø)
.../src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	99.09% <0.00%> (+0.24%)	⬆️
...ingRequestsCollectionsIsAccordingToTheSpecTests.cs	95.91% <0.00%> (+2.04%)	⬆️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.95% <0.00%> (+3.17%)	⬆️

[eddynaka]

It's a nice improvement!

nit: we should somehow try to merge all this functions to one, since all looks the same...not sure if that's feasible

[CodeBlanch]

@eddynaka I combined ActivityTagObjectsEnumeratorFactory & ActivityTagsCollectionEnumeratorFactory into ActivityTagsEnumeratorFactory. So now there are 3 factories: 1 for tags (activity.TagObjects, activityLink.Tags, & activityEvent.Tags), 1 for events (activity.Events), & 1 for links (activity.Links). How's that?

[eddynaka]

LGTM