Jaeger-collector downsampling before enqueue instead of after dequeue.

[jianwu]

Seems the current collector down sampling happens after dequeuing from the collector in-memory queue before saving to storage. Any reason not to do it before enqueue? That will protect the in-memory queue from overflowing by the misbehaved clients and it the collect more resilient for the spike of traffic. (Less chance of overflow as fewer spans are enqueued).

[yurishkuro]

agreed, makes more sense before enqueueing