Returns a single PauliMeasurementGate which measures the pauli observable

pauli_observable The cirq.PauliString observable to measure.
key Optional str or cirq.MeasurementKey that gate should use. If none provided, it defaults to a comma-separated list of the target qubits' str values.

An operation measuring the pauli observable.

ValueError if the observable is not an instance of PauliString.