SQL Expressions: Rework backend errors and error instrumentation (#109633)

* Capture error_type label on metrics/traces
* Make error messages more helpful to user
* Use errutil, categorized errors, and tie them to error_type (category in code)
* Misc trace fixes
* Add metric to track SQL input conversion

pkg/expr/convert_to_full_long.go

@@ -27,6 +27,7 @@ func ConvertToFullLong(frames data.Frames) (data.Frames, error) {
 	if frames[0].Meta != nil && frames[0].Meta.Type != "" {
 		inputType = frames[0].Meta.Type
 	} else {
+		// shouldn't hit this when calling from handleSqlInput as supportedToLongConversion is called first
 		return nil, fmt.Errorf("input frame missing FrameMeta.Type")
 	}
 
@@ -40,6 +41,7 @@ func ConvertToFullLong(frames data.Frames) (data.Frames, error) {
 	case data.FrameTypeTimeSeriesWide:
 		return convertTimeSeriesWideToFullLong(frames)
 	default:
+		// Shouldn't hit this when calling from handleSqlInput as supportedToLongConversion is called first
 		return nil, fmt.Errorf("unsupported input type %s for full long conversion", inputType)
 	}
 }