OperationTypeInsert inserts a new k/v if it isn't already present.

OperationTypeUpdate only modifies an existing k/v.

OperationTypeUpsert does both of above.

TODO: It'd be cleaner to get these from the otel semver package? Not all are in semver though.

TODO: Put a private member so that this can't be created without calling NewConsumer?.

Options configure a Consumer.