xds/internal/xdsclient: Process string metadata in CDS for com.google.csm.telemetry_labels

[zasweq]

This PR adds processing in CDS for string telemetry labels with filter_metadata string "com.google.csm.telemetry_labels". Other value types and filter_metadata identifiers are ignored. This also does not introduce NACK's. This is based off the C-Core implementation, which I disagree with technically but understand there were good reasons for the decisions made. See discussion in xDS Chat room for context as to why C-Core did it this way and why I followed it.

The relevant language in the CSM Observability Design is "The above key_value pairs from com.google.csm.telemetry_labels are going to be parsed by xDS Client from the CDS Resource and propagated to the watcher on this resource in the “cds” load balancing policy."

RELEASE NOTES: N/A

[codecov]

Codecov Report

Merging #7085 (dcc290c) into master (4ec8307) will decrease coverage by 1.36%.
Report is 7 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7085      +/-   ##
==========================================
- Coverage   82.54%   81.19%   -1.36%     
==========================================
  Files         305      345      +40     
  Lines       31393    33940    +2547     
==========================================
+ Hits        25913    27557    +1644     
- Misses       4425     5212     +787     
- Partials     1055     1171     +116     

Files	Coverage Δ
...ds/internal/xdsclient/xdsresource/unmarshal_cds.go	86.19% <100.00%> (+0.41%)	⬆️

... and 65 files with indirect coverage changes

[dfawley]

This feels weird. We check the type of a thing, then call GetStringValue on some other thing. Does GetStringValue do something different if the Kind() is not StringValue?

[zasweq]

Yeah, this is the best solution that I could come up with. GetStringValue() returns a string, not a *string, so there is no way to distinguish between an empty string (a valid label value, which will be recorded) and an unset string (should eventually record method labels as "unknown" if unset). Thus, I don't set if not a string value (not set = "unknown" in OpenTelemetry component) and set it even if it's empty if it's present.

[dfawley]

I believe you can do:

if s, ok := val.AsInterface().(string); ok {
	stringMD["service_name"] = s
}

But I'm not sure that that's much better.

[zasweq]

There's no function on the val called AsInterface() :/. Also, I think that typecast is wrong because it's a wrapper interface which happens to contain the value string. I.e. there's another layer (not 100% sure on this, but pretty sure).