Closes #14357.

Adds a new prometheus metric coderd_agentstats_currently_reachable_peers that includes the current number of pingable peers. The label connection_type can currently be either derp or p2p.

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 agent main on the workspace workspace1 using the docker 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 these agentstats metrics to be exposed on coderd.