enhancement(prometheus_remote_write source): Reintegrate complex metrics (#6280)

* Add support for parsing prometheus text timestamps

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Rename OtherMetric to SimpleMetric

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Move histogram and summary grouping into prometheus-parser

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Rename prometheus sink parse to parse_text and simplify

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Move prometheus protobuf definitions into prometheus-parser lib

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Move much of MetricGroup functionality into GroupKind

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Split up the metric group key (timestamp, label) from the data

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Add support for parsing remote_write requests

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Reparse groups directly into events in prometheus source lib

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Wire in the new parser lib into the remote_write source

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Break metric finisher out of MetricBuffer::finish iterator

Signed-off-by: Bruce Guenter <bruce@timber.io>