Define and implement an initial set of telemetry metrics for the agentchannel and the sshchannel services

• number of registrations received / hosts added
• number of deregistrations received / hosts removed
• number of currently registered agents / available hosts
• number of received environment requests
• number of offers produced
• number of busy agents / hosts (updown)
• number of idle agents / hosts (updown)
• number of unreachable agents / hosts (updown)
• per tag, number of registered agents / available hosts (updown)