Data
The type for timestamp. A timestamp shows the date and time, in RFC3339 UTC, associated with particular data.
timestamp t is t's timestamp (if any). If it is None, then the reporter will add a new timestamp automatically.
v ?timestamp f is the measure f, as a the list metric name and value, and the timestamp timestamp. If timestamp is not provided, it will be set be the reporter. Raise Invalid_argument is a key or a value contains an invalid character.