[cmd/mdatagen] only import "slices" when a required reagg attribute exists

The config.go.tmpl unconditionally imports "slices" whenever
reaggregation is enabled and there are aggregatable (recommended/opt_in)
attributes. However, slices.Contains is only emitted for *required*
attributes. When a receiver has reaggregation enabled but carries no
required attributes, the generated file fails to compile with
"slices imported and not used".

Add a second template variable $hasRequiredReagAttr and gate the
"slices" import on it.

Signed-off-by: Ali <alliasgher123@gmail.com>

Author: alliasgher

cmd/mdatagen/internal/templates/config.go.tmpl

@@ -16,6 +16,7 @@ import (
 	{{- if $hasReagMetricsWithRequired }}
 	"slices"
 	{{- end }}
+	{{- end }}
 
 	"go.opentelemetry.io/collector/confmap"
 	{{- if and .Metrics .ResourceAttributes }}