Added Activity.EnumerateLinks extension

[CodeBlanch]

Added an extension method for enumerating Activity Links without an allocation. Updated Jaeger to use it.

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
EnumerateNonemptyActivityLinks	94.488 ns	0.6160 ns	0.5762 ns	0.0076	-	-	64 B
EnumerateLinksNonemptyActivityLinks	48.164 ns	0.4717 ns	0.4181 ns	-	-	-	-

TODOs:

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

[CodeBlanch]

I switched IActivityTagEnumerator to IActivityEnumerator<T> so I could use it for tags, links, and events (up next). Breaking change, which is a con, but pro is with this change we don't need a bunch of interfaces that are basically the same, just different item types.

[codecov]

Codecov Report

Merging #1314 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1314      +/-   ##
==========================================
+ Coverage   78.51%   78.63%   +0.11%     
==========================================
  Files         219      219              
  Lines        6243     6253      +10     
==========================================
+ Hits         4902     4917      +15     
+ Misses       1341     1336       -5     

Impacted Files	Coverage Δ
...plementation/ZipkinActivityConversionExtensions.cs	92.07% <ø> (ø)
src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	98.59% <100.00%> (+0.40%)	⬆️
....Jaeger/Implementation/JaegerActivityExtensions.cs	96.20% <100.00%> (+0.47%)	⬆️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	100.00% <0.00%> (+2.85%)	⬆️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.95% <0.00%> (+3.17%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️

[CodeBlanch]

@eddynaka @cijothomas Just updated branch and I'm getting a strange, seemingly unrelated, test failure:

X OpenTelemetry.Tests.EventSourceTest.EventSourceTest_OpenTelemetrySdkEventSource [25ms]
  Error Message:
   System.Exception : Method 'OpenTelemetrySdkEventSource.MetricObserverCallbackError' is implemented incorrectly.

Any idea what might be causing that?

[eddynaka]

@eddynaka @cijothomas Just updated branch and I'm getting a strange, seemingly unrelated, test failure:

X OpenTelemetry.Tests.EventSourceTest.EventSourceTest_OpenTelemetrySdkEventSource [25ms]
  Error Message:
   System.Exception : Method 'OpenTelemetrySdkEventSource.MetricObserverCallbackError' is implemented incorrectly.

Any idea what might be causing that?

It's an order issue. I couldn't find a way to trigger. but sometimes happens.