mdatagen: generates code that's missing a dependency

[codeboten]

The groupsbyattr and filter processors are missing the "go.opentelemetry.io/otel/metric" import in the code that mdatagen generates for them. This causes an issue as go fmt will then try to pull in the dependency and it pulls in "go.opentelemetry.io/otel/sdk/metric" dependency instead of "go.opentelemetry.io/otel/metric"