client: log difference detail when client fingerprinter detects a change

[jrasell]

The Nomad client fingerprinters are responsible for gathering information about the host system and external APIs that the Nomad client will utilise, such as Consul and Vault. This information is gathered and sent to the Nomad servers via the Node.Register RPC and written to the FSM, so the information can be stored in state and used during scheduling calculations.

Some fingerprinters such as Vault implement the Periodic function, which means they run continually on an interval.

In the event a periodic fingerprint detects changed attributes from its previous run, the RPC will be sent. Monitoring systems may identify this via RPC or FSM monitoring, however, understanding the reason for the RPC and FSM event is tricky and requires manual identification. It would be useful to have a client log message to indicate its fingerprinter attributes have changed, and what has changed, so operators can easily see this information. It may also be useful to emit a telemetry datapoint when change has occurred, so the rate across a cluster can be alerted on, when it reaches a threshold indicating too much churn/flapping is happening.