Prior to this commit, metrics instrumentation was provided by Spring
Boot, using Micrometer metrics. The `Instrumentation` would publish two
timers and a counter, but would not support tracing.
This commit replaces the former with a dedicated support in Spring for
GraphQL directly. This uses the new `Observation` API from Micrometer
and publishes two observations:
* a request execution observation, with timing and tracing included
* a data fetching observation with the request execution as a parent
observation
Closes gh-501
