Support `http/json` in `otlpmetrichttp`

[petern48]

Problem Statement

otlpmetrichttp does not support http/json encoding format.

Proposed Solution

Add support for encoding in protojson format and make it configurable, keeping proto as the default option.

Alternatives

Prior Art

Additional Context

open-telemetry/opentelemetry-go-contrib#6339
open-telemetry/opentelemetry-go-contrib#8399

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[petern48]

/assign