Update swagger for unstable OtelMetricsIngestion

[enisoc]

Note that I made some changes under tools/ to prevent otel-metrics-ingestion from being pluralized despite being a singleton. It appears that all the previous singletons had base entity names that were themselves plural, such as resource-pools and trace-tail-sampling-rules, because the singleton object semantically represented a list of multiple things (pools/rules).

By contrast, otel-metrics-ingestion is a pure singleton config that does not represent a list of anything, so it never makes sense to pluralize the name. Please let me know if there's a better way to achieve this.