Add metric to track number ssh connect attempts

[rcanderson23]

This adds the metric teleport_connect_to_node_attempts_total

# HELP teleport_connect_to_node_attempts_total Number of ssh connect attempts
# TYPE teleport_connect_to_node_attempts_total counter
teleport_connect_to_node_attempts_total 4

This is added so that we can track percentage of successful connections as we already track failed attempts with failed_connect_to_node_attempts_total

[zmb3]

I'm accustomed to seeing related metrics like this have names that are more similar.

I recognize we're limited by the original metric, but ideally we would have something like:

teleport_connect_to_node_attempts_failed
teleport_connect_to_node_attempts_total

Since we don't want to change the original metric, I wonder if we could at least update the Help field or the docs so that these metrics mention each other.

[rcanderson23]

Updated the Help on both metrics as well as the docs to point to each other for usage and indicate they are linked.

[ptgott]

Left a couple of very minor nits.