statsd: support sending metrics with timestamp.

[remeh]

Adds two methods GaugeWithTimestamp and CountWithTimestamp in the ClientInterface to support sending gauge and count with timestamp, for users aggregating metrics on their side and who can provide a timestamp for a given metric.

Metrics sent using GaugeWithTimestamp and CountWithTimestamp to the DogStatsD server will be directly sent to the intake with no aggregation done server-side.

If the client is created using WithClientSideAggregation(), no aggregation will be performed on metrics submitted with timestamp, they will be written in the serialization buffer as-is.