Some metrics-related beans are only created when using Java 7

[wilkinsona]

Beyond the Java 8-specific implementations, there are some differences in the metrics-related beans that are created when using Java 7 versus Java 8:

• InMemoryMetricRepository is only created when using Java 7 but the javadoc states that "An InMemoryMetricRepository is always created unless another MetricRepository provided by the user"
• An @ActuatorMetricWriter-qualifed bean is only created when using Java 7

This relates to #6982 as it's not clear what should and should not be removed for Boot 2.0 where we require Java 8.

@dsyer Can you remember what the intention was here?

At the least, I think we need to update the javadoc for the auto-configuration.

[eddumelendez]

I just was working on that and stop there.

[joshiste]

I think it somehow relates to PR #4951 (which should better be an issue...)

[dsyer]

I think the javadoc is just mouldy (it describes the behaviour prior to the Java 8 support being added). Maybe there needs to be an extra bean in Java8 (and also with Dropwizard if I catch @joshiste's drift) so there is always a @ActuatorMetricWriter?

[wilkinsona]

Following a helpful chat with @dsyer I now have a much better understand of the intent here. InMemoryMetricRepository is, largely, an internal of the Actuator's metrics support. Application code should deal with GaugeService and CounterService, both of which are present with both Java 7 and Java 8. The javadoc needs to be updated on MetricRepositoryAutoConfiguration to reflect this.