Add KEDA-scaling for Sources

[nachocano]

Problem
Our PubSub-based Sources are Pull-based. Given that our receive adapter polls for new messages, it’s hard to decide when to scale it out. In other words, the draining of the PubSub subscriptions will eventually become a throughput bottleneck.

There are some initial talks in the Sources WG regarding Pull-based sources and how to scale them. @aslom suggested KEDA. Either with KEDA or something else, we will need to read metrics from Stackdriver to see how “congested” the subscription is and inform the scaling mechanism. See details on metrics here.

Exit Criteria
Being able to scale out our receive adapters.

Time Estimate (optional):
How many developer-days do you think this may take to resolve?
2-3 weeks

[nachocano]

As pointed out by @yolocs , KEDA seems to already be using SD metrics to scale PubSub... https://github.com/kedacore/keda/blob/master/pkg/scalers/gcp_pub_sub_scaler.go#L139
Thanks @yolocs for the pointer.

[nachocano]

/assign @ian-mi
in case you are interested, this can have a high impact... and I'd be interested in helping out...

[nachocano]

WIP on the kafka source knative/eventing-contrib#886

[nachocano]

Starting working on some of this to see if it would work... This will require quite some work to have something generic enough for other sources to benefit...
fyi @nlopezgi, we can talk more offline

[nachocano]

Integration done, seems to be working for the different sources.
Still needed to add UTs and documentation