ContainerAge in ContainerMetric

[OzJacobii]

Please add ContainerAge value to the ContainerMetric
https://docs.pivotal.io/platform/application-service/2-8/loggregator/container-metrics.html

[antechrestos]

@OzJacobii I use dropsonde protocol specification and indeed it misses some attributes.

I made a pull request to upgrade it.
Let's see if it gets accepted. 🙏