[Tag value search] Support q query param in /api/v2/search/<tag.name>/values

[mapno]

What this PR does:

Add support for passing a traceql query to tag values search, via query param q. The query can be incomplete—eg. { .cluster = "foo" && .namespace = }. In such cases, Tempo will extract the value matchers, ie. foo = "bar", and build a query that can be parse by the storage layer.

This is done with a regex that captures the valid matchers. Below is a benchmark of this function for some queries. Overall, the cost of extracting the matchers is very low, and shouldn't affect latency.

go test -bench=BenchmarkExtractMatchers -run=^$ ./pkg/traceql
goos: darwin
goarch: amd64
pkg: github.com/grafana/tempo/pkg/traceql
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
BenchmarkExtractMatchers/{.service_name_=_"foo"}-8                                                                                                 	  786459	      1482 ns/op
BenchmarkExtractMatchers/{.service_name_=_"foo"_&&_.http.status_code_=_200}-8                                                                      	  405924	      2781 ns/op
BenchmarkExtractMatchers/{.service_name_=_"foo"_&&_.http.status_code_=_200_&&_.http.method_=_"GET"}-8                                              	  301669	      3908 ns/op
BenchmarkExtractMatchers/{.service_name_=_"foo"_&&_.http.status_code_=_200_&&_.http.method_=_"GET"_&&_.http.url_=_"/foo"}-8                        	  450922	      2730 ns/op
BenchmarkExtractMatchers/{.service_name_=_"foo"_&&_.cluster_=_}-8                                                                                  	  453796	      2488 ns/op
BenchmarkExtractMatchers/{.service_name_=_"foo"_&&_.http.status_code_=_200_&&_.cluster_=_}-8                                                       	  297260	      3895 ns/op
BenchmarkExtractMatchers/{.service_name_=_"foo"_&&_.http.status_code_=_200_&&_.http.method_=_"GET"_&&_.cluster_=_}-8                               	  232723	      5048 ns/op
BenchmarkExtractMatchers/{.service_name_=_"foo"_&&_.http.status_code_=_200_&&_.http.method_=_"GET"_&&_.http.url_=_"/foo"_&&_.cluster_=_}-8         	  454668	      2646 ns/op
PASS
ok  	github.com/grafana/tempo/pkg/traceql	9.972s

Which issue(s) this PR fixes:
Contributes to #1868

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[knylander-grafana]

Do we need to add docs for this?

[mapno]

Do we need to add docs for this?

Yes. I will update the API docs shortly 👍

[mdisibio]

Let's change selector to spanset filter which matches the traceql definitions, and add a few examples of queries that match or don't match.

[mdisibio]

Since wantAttr.Scope doesn't change, we should move this switch logic out of the loop, it can be done once before the Filter function.

[mdisibio]

We can return io.EOF to completely stop the Fetch.

[mdisibio]

LGTM - Left a few comments, which we discussed offline.