Add elapsedTime tracking to Timer metrics

[donbourne]

see discussion in #496

[Channyboy]

Will need to be marked for 3.0.
The updated interface for Timer throws an error from the bnd-baseline-maven-plugin plugin.

[jmartisk]

@Channyboy isn't that because we need to update the bundle version in package-info.java files?
https://github.com/eclipse/microprofile-metrics/blob/2.4-M1/api/src/main/java/org/eclipse/microprofile/metrics/package-info.java
and
https://github.com/eclipse/microprofile-metrics/blob/2.4-M1/api/src/main/java/org/eclipse/microprofile/metrics/annotation/package-info.java

I think it will work then after changing them to 2.4. This is not a reason for doing a major version bump

[donbourne]

I believe the problem is that if we add more methods to the Timer interface then any app-built implementations of Timer would break.

[jmartisk]

You're right it seems. We can go ahead with this only if we actually bump to 3.0.

[jmartisk]

Then I guess the same issue will be with #523
Perhaps we should consider doing a major bump after all, given the opportunity that we will have now