[receiver/wavefront] Migrate from OpenCensus data model to pdata

[dmitryax]

The receiver still uses OpenCensus data model to create the metrics and converts them into pdata before sending them to the next consumer. This introduces a performance impact. The receiver should use pdata model to create the metrics instead.

It depends on #20759. Once the carbon receiver is migrated, only tests need to be updated in this receiver.

[github-actions]

Pinging code owners for receiver/wavefront: @pjanotti. See Adding Labels via Comments if you do not have permissions to add labels yourself.

[github-actions]

Pinging code owners:

• receiver/wavefront: @pjanotti

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[github-actions]

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Pinging code owners:

• receiver/wavefront: @pjanotti

See Adding Labels via Comments if you do not have permissions to add labels yourself.