[TraceQL Metrics] Fix handling of empties and nils + other fixes

[mdisibio]

What this PR does:
Basically this overhauls the handling of timeseries labels for metrics queries. There are several fixes in here, but it also lines up behavior with: grafana/grafana#83619

• Empty strings were previously dropped but now they are preserved. For example: {rootName=""} | rate() by (rootName). This would look like a completely empty labelset in the response and cause issues downstream. Now it's preserved as the single label rootName with an empty string value.
• When all labels are nil, we were backfilling a single label with the text "" to make Grafana happy like. This was actually premature specialization for the prometheus-compatible API, and now that we have the new dedicated Tempo-UI we can fix this. Now nils are dropped entirely and handled in the frontend. They will become {} in the Tempo UI and the query name in the Prometheus UI (a fallback method). Note: the EmitDefaults:true is part of this so that Tempo always outputs an empty array in the json (and not nil).
• Fix sorting. Previously we only sorted during the conversion to Prometheus format. Also fix sorting to handle non-strings and multiple label values correctly.
• Fix default step calculation and error handling.

Which issue(s) this PR fixes:
Fixes #3429

Checklist

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

[mdisibio]

Requires: grafana/grafana#83619

[mapno]

LGTM