[Feature Request] send only on value changes

[toni-moreno]

Could be interesting implement a new metric type which only sends value changes as sugested in influxdata/influxdb#7739.

[toni-moreno]

In addition this "status" fields where could be interesting send only changes.
There is some other common configuration (at least on net devices as switches).

depending on the metric meaning it could be configured Report as:

1. ALWAYS_SEND: send if value exist with the exact value. ( bandwing metrics by example)
2. NEVER_SEND: these metrics has sense only when this value act as input for other "STRING-EVAL" metrics.
3. SEND_ON_CHANGE: send data only when the value has changed, has sense on "status" fields ( examples on the network iFStats are ifAdminStatus/ifOperStatus.
4. SEND_ON_NOTZERO: send data only when values are different of zero, typical on error counters (once difference has been computed)