Add operator initialization metric to aggregator

[Hyodar]

Adds a simple metric to track operator initializations in aggregator. The idea here is not that we strictly need to know that operator X has initialized, but at least have some kind of ping so we can detect crashes if they happen.

[emlautarom1]

LGTM

I don't see anything worth mentioning except for the usage of logger.Errof instead of logger.Error with tags (I'm not sure if there's an agreement on what to use)

[Hyodar]

Oh well. Yeah, we've not settled on one specifically, but we've been trying to use Error + tag "err". I see there are multiple places in metrics that the format one is used, so I don't think this is work for this PR. Added an issue for that: #179.