Add support for prometheus push gateway to indexer

[mjpitz]

Should be pretty easy to hook into this. All the client configurations use the default registry. See the easy push example in the link here:

https://godoc.org/github.com/prometheus/client_golang/prometheus/push

In addition to adding support for the push gateway, let's also add some metrics to understand where failures are happening in our indexing funnel. This will require the addition of some new custom metrics.