Automatically flush for > 100 metrics per key

[jaredcnance]

Currently, if put_metric is called > 100 times, it will fail silently on the backend. We should automatically flush client-side if this limit is hit.

[jchangxu]

The previous PR resolve the case when the Unqiue Metrics are over 100, but it does not resolve the case, when the Metrics values are over 100.