chore(observability): Track all Fence feature flags in DataDog

[themarcelor]

We should be able to track / filter all feature flags across all environments by submitting the microservice configurations as metric tags, e.g., https://app.datadoghq.com/dashboard/r8v-5v7-t6q?from_ts=1632151862728&to_ts=1632155462728&live=true

This should help us "dark launch" features without exposing the config in the cdis-manifest repo. Once this is in dataDog, all those booleans can easily queryable / filterable in a dashboard. We can obtain insights, such as: “WE have 40+ websites.. in which ones is audit-service enabled?” or “We are trying some A/B testing in several staging environments for a new GA4GH feature.. where is it enabled across a list of 20 environments? What is the latency / error/ saturation / traffic on the envs where feature_flag X Y Z is enabled?”