[processor/transform] Add metrics data model

[TylerHelmuth]

Description:
Adds the metrics data model to the transform process. Wiring up the processor to the metrics pipeline will be a different PR. Similar to #9271, logic has been stolen from traces and logs. Similar logic will be broken out later. Within metrics.go itself there is a lot of duplicate logic (especially with the switch statement). I am hoping generics can help with this in the future.

Link to tracking Issue:
#8252

Testing:

Documentation:

[TylerHelmuth]

Open Questions:
1. How to handle accessing individual items within exemplars and quantile_values.
2. How to access DataType fields such as is_monotonic and aggregation_temporality

[bogdandrutu]

/cc @anuraaga

[TylerHelmuth]

2. How to access DataType fields such as is_monotonic and aggregation_temporality

Handled via the descriptor. can access via descriptor.metric_aggregation_temporality and descriptor.metric_is_monotonic

[TylerHelmuth]

1. How to handle accessing individual items within exemplars and quantile_values.

I think we'll have to handle exemplars and quartile_values either via a new transformContext so that they can have their own newPathGetSetter or only ever as a list, and functions that want to mess with individual items will need to receive the whole list and then make decision. For this PR, I'll be opting on the latter, which is also the existing pattern for Events and Links.

[TylerHelmuth]

Added tests for NumberDataPoint, HistogramDataPoint, ExponentialHistogramDataPoint, SummaryDataPoint, and Descriptor. I did not add tests for Resource or InstrumentationScope since the code is the same as traces and logs, which have tests, and we are planning to refactor both into a single place.

[TylerHelmuth]

@anuraaga ready for official review

[anuraaga]

Thanks! Just suggestion on the new method on TransformContext

[TylerHelmuth]

@open-telemetry/collector-contrib-approvers please review.

[dehaansa]

Wired this up into a full processor locally to test out a use case we're looking for (resource attribute copied down to metric attribute) and it worked perfectly. 👍

[djaglowski]

LGTM

[djaglowski]

@TylerHelmuth should this have a changelog entry?

[TylerHelmuth]

@TylerHelmuth should this have a changelog entry?

I didn't add one bc #9271 didn't have one. A changelog entry was added when logs were wired up (#9368)