Update documentation for metrics send and send_all with all metric types

[PatrykMilewski]

Right now documentation says, that for Metrics send_all and send functions, we can pick one of 2 metric types:
metric_type|type: the type of metric to use ("gauge" or "count") [default: gauge]
and it's no longer true.

Since DataDog supports metric type "rate" and this library doesn't do any type validation, "rate" metric type is correctly supported by those functions.

Documentation for Metrics send_all and send should be:
metric_type|type: the type of metric to use ("gauge", "count" or "rate") [default: gauge]