[processor/transform] Add metrics data model (open-telemetry#9719)

* Started adding metrics data model

* Add metric descriptor

* Switched item to datapoint

* Added all NumberDataPoint fields

* Adding all fields for histograms and summary

* run gotidy

* added agg_temporality and is_monotonic

* Add NumberDataPoint tests

* Add HistogramDataPoint tests

* Add ExpoHistogramDataPoint tests

* Add SummaryDataPoint tests

* Add Descriptor tests

* removed IL tests

* Fix unit test

* Fix lint

* removed GetDescriptor function and switched to cast

* switch from descriptor to metric