Can't find "broker_filter_event_latencies" in prometheus

[capri-xiyue]

Describe the bug

Can't find metrics "broker_filter_event_latencies" in prometheus("event_latencies" in pkg/broker/filter/stats_reporter.go)

I found it's because we only suppor *string in the ExtensionAs() of eventcontext.

Here is the error log messgage: event extension is map[knativearrivaltime:2019-08-28T18:43:22.986315189Z knativehistory:default-kn2-trigger-kn-channel.default.svc.cluster.local]
extError is unkown extension type *time.Time{"level":"debug","ts":"2019-08-28T18:43:24.986Z","logger":"provisioner","caller":"filter/filter_handler.go:176","msg":"Received message","commit":"96d090e","triggerRef":"default/testevents-trigger"}.

Expected behavior

Be able to see "broker_filter_event_latencies" metrics in prometheus

To Reproduce

Need to add some logs when we try to get arrival time in pkg/broker/filter/filter_handler.go(event.ExtensionAs(broker.EventArrivalTime, &arrivalTime)

Knative release version

Additional context
Add any other context about the problem here such as proposed priority

[Harwayne]

As a potential solution, we can store the time in a known format in the CE attribute, then parse that back into a time.Time in the filter.

[nachocano]

/assign