[pkg/otlp/metrics] Add WithInitialValueMode to metrics translator

[mx-psi]

What does this PR do?

• [pkg/otlp/metrics] Use NumberMode internally instead of 'SendMonotonic'
• [pkg/otlp/metrics] Add initial value mode to set what to do with the initial value of a cumulative monotonic sum when under the 'cumulative_to_delta' mode. Possible values are the same as in [processor/cumulativetodelta] make first point drop configurable open-telemetry/opentelemetry-collector-contrib#21905.

Motivation

Relates to #100

[mx-psi]

The breaking changes report is reporting a change on a private struct:

Changes for github.com/DataDog/opentelemetry-mapping-go/pkg/otlp/metrics:
Incompatible changes:
- translatorConfig.SendMonotonic: removed
Compatible changes:
- InitialValueMode: added
make: *** [Makefile:8[6](https://github.com/DataDog/opentelemetry-mapping-go/actions/runs/5357793420/jobs/9719189185?pr=109#step:8:7): apidiff-compare] Error 1
- InitialValueModeAuto: added
- InitialValueModeDrop: added
- InitialValueModeKeep: added
- WithInitialValueMode: added
- translatorConfig.InitialValueMode: added
- translatorConfig.NumberMode: added
Incompatible changes found!

While this could be a breaking change if the struct was embedded somewhere, it is not the case. I am going to add the breaking-changes label to skip the check as this is a false positive.