Remove statsd exporter usage completely once Enovy stats endpoint supports histograms

[tiffanyfay]

Here's some Envoy histogram support that was merged: envoyproxy/envoy#3130 (review)

Envoy emits three types of values as statistics:
Counters: Unsigned integers that only increase and never decrease. E.g., total requests.
Gauges: Unsigned integers that both increase and decrease. E.g., currently active requests.
Histograms: Unsigned integers that are part of a stream of values that are then aggregated by the collector to ultimately yield summarized percentile values. E.g., upstream request time.