Closes#14357.

Adds a new prometheus metriccoderd_agentstats_currently_reachable_peers that includes the current number of pingable peers. The labelconnection_type can currently be eitherderp orp2p.

For example:

coderd_agentstats_currently_reachable_peers{    agent_name="main",    connection_type="derp",    template_name="docker",    username="admin",    workspace_name="workspace1"} 1

Reflects that the agentmain on the workspaceworkspace1 using thedocker template has one peer, and that peer is connected over DERP.

Additionally, updates the documentation to make it clear that--prometheus-collect-agent-stats must be set for theseagentstats metrics to be exposed oncoderd.