[extension][ecsobserver] Init ECS Observer README (#2377)
[extension][ecsobserver] Init ECS Observer README (#2377)

* ecsobserver: Init README

* ecsobserver: Update output format to use __meta_ecs_ prefix

* ecsobserver: Fix typo

- `make misspell`
- Fix order of filter types, should be service, task defintion, docker
  label
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib