Added GetTagValue Activity Extension

[CodeBlanch]

All the struct enumerators I worked with the .NET team to introduce are available in RC1 so now we can have some fun with the perf.

Changes

This adds a GetTagValue extension on Activity for finding a tag value by key very efficiently.

Benchmarks

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
EnumerateEmptyTagObjects	8.776 ns	0.1528 ns	0.1355 ns	-	-	-	-
LinqEmptyTagObjects	21.909 ns	0.4546 ns	0.4465 ns	-	-	-	-
GetTagValueEmptyTagObjects	1.826 ns	0.0335 ns	0.0297 ns	-	-	-	-
EnumerateNonemptyTagObjects	69.014 ns	1.3853 ns	1.2958 ns	0.0048	-	-	40 B
LinqNonemptyTagObjects	89.760 ns	1.6389 ns	1.5331 ns	0.0048	-	-	40 B
GetTagValueNonemptyTagObjects	49.510 ns	0.2616 ns	0.2042 ns	-	-	-	-

For significant contributions please make sure you have completed the following items:

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

[codecov]

Codecov Report

Merging #1221 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   78.76%   78.81%   +0.04%     
==========================================
  Files         219      219              
  Lines        6311     6329      +18     
==========================================
+ Hits         4971     4988      +17     
- Misses       1340     1341       +1     

Impacted Files	Coverage Δ
src/OpenTelemetry/Internal/EnumerationHelper.cs	91.42% <ø> (ø)
src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	97.43% <100.00%> (+2.19%)	⬆️
...try.Instrumentation.GrpcNetClient/GrpcTagHelper.cs	100.00% <100.00%> (ø)
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
...us/Implementation/PrometheusExporterEventSource.cs	72.72% <0.00%> (+9.09%)	⬆️

[eddynaka]

Don't know how hard is, but we could add another test using Expression.Lambda and compare the perf. The same that we did when we were trying to create the SetKind. What do you think?

[reyang]

Curious - I wonder if there are many scenarios where people would try to get multiple tag values. If yes, a further improvement might be giving a GetTagValues overload (which takes an array/list of names and a ref to Span or something allocated on the stack).

[CodeBlanch]

There's actually a method a few lines up doing two lookups! 1 for status code, 1 for status description. I'll take a stab at this later. If the PR gets merged I'll do as a follow-up.

[reyang]

LGTM!

[CodeBlanch]

Update: I found the spots using Linq in the main solution and updated them to use the extension. So it is now providing immediate value. I was looking for TagObjects, they were using Tags. 🙄

[CodeBlanch]

@eddynaka

Don't know how hard is, but we could add another test using Expression.Lambda and compare the perf. The same that we did when we were trying to create the SetKind. What do you think?

I tried, and it is really hard! The one we did for setting kind on Activity, we knew both types (Activity & ActivityKind). The enumeration stuff is a whole different ballgame because you don't know the concrete type or the enumerator type it uses. All you really know is it is an IEnumerable<T>. In the case of TagObjects, it uses an internal linked list thing that is not public. The enumerator type is internal too.

I'm not convinced it is impossible to do using expression syntax, it's just going to take some time to figure it out. If you have any ideas please share! Maybe on Gitter so we don't spam this?