SpringBoot Update Dynatrace Micrometer registry documentation

We introduced a few improvements to the Dynatrace meter registry for Micrometer from version 1.9.0. This PR updates the documentation to reflect these changes.

I think this should also be merged into the 2.7.x branch (since that branch uses Micrometer 1.9.0). Is there anything I need to do to merge it to that branch as well?

Comment From: philwebb

Thanks @pirgeo. There's not need for a second PR, we can cherry-pick this update to 2.7.x then merge forward.

Comment From: philwebb

Thanks @pirgeo, this is now in 2.7 and 3.0

Comment From: pirgeo

Thanks a lot!