add metrics for message aggregator

[#75143014]

Signed-off-by: Georg Apitz <georg@pivotallabs.com>
integrate message aggregator with metron

[#75143014]

Signed-off-by: Georg Apitz <georg@pivotallabs.com>
fix flaky test

[#75143014]

Signed-off-by: Georg Apitz <georg@pivotallabs.com>