In the scope of the broader issue #1312, this PR deals with performing
those changes for the `graph` service, namely to add the ability to
disable the HTTP API or to disable the events API handler by
configuration.
It also adds metrics for the events processing, and tests for the events
processing.
The previous implementation was combining the HTTP server service and
the events consumption, which is why this PR refactors the composition
of those services:
* the event consumption has been moved into its own service
* the identity.Backend is created beforehand, and then injected as a
collaborator in both the HTTP service as well as the event consumer
service
It also adds metrics, mainly for the event processing.
To encourage re-use in latter implementations and changes, it also
introduces two top-level package changes:
* internal/eventstest/events_test_helpers: contains a TestBus
implementation to unit-test event consumers without NATS
* internal/metricstest/metrics_test_helpers: contains assertion
functions to test Prometheus metrics
