[TraceQL] Add topk/bottomk to TraceQL Metrics

[joe-elliott]

TraceQL metrics brings the promise of querying infinite cardinality, but in a practical sense both Tempo and Grafana suffer once we start returning series in the 10s of thousands. It can also be quite difficult to parse this amount of information looking at a time series graph.

Add topk/bottomk style functionality to the language to give users control over cardinality and return the series they are most interested in. Syntax would look something like:

{} | rate() by (span.client_ip) | topk(10)