fix(datadog_metrics sink): improve aggregation performance (vectordotdev#18759)

* fix(datadog_metrics sink): improve aggregation performance

* fix tests and add concurrency

* feedback

* add comment to dedupe_by

* sort by metric type as well

* fix flakey proptest

* only dedupe non-counters in prop test

Author: dsmith3197

regression/cases/syslog_regex_logs2metric_ddmetrics/vector/vector.toml

@@ -27,7 +27,7 @@ type = "log_to_metric"
 inputs = ["remap"]
 
   [[transforms.log2metric.metrics]]
-  type = "gauge"
+  type = "counter"
   field = "procid"
   tags.hostname = "{{ hostname }}"
   tags.facility = "{{ facility }}"