Add histogram to keep track of node healthcheck latency

We need a histogram of healthcheck latencies so we can alert when this latency begins to drop. This is a proxy for gitaly node saturation.

This MR adds:

• a HistogramVec interface
• a function for registering the histogram for node latencies
• inject this histogram into the node manager
• call Observe on the histogram whenever we call a node's healthcheck

fixes: #2250 (closed)