It was calculating the number of dropped time series instead of the number of dropped samples.
While at it, drop vmalert_remotewrite_dropped_bytes_total metric, since it was inconsistently calculated -
at one place it was calculating raw protobuf-encoded sample sizes, while at another place it was calculating
the size of snappy-compressed prompbmarshal.WriteRequest protobuf message.
Additionally, this metric has zero practical sense, so just drop it in order to reduce the level of confusion.
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