Output only. The confidence estimate between 0.0 and 1.0. A
higher number indicates an estimated greater likelihood that
the recognized words are correct. This field is set only for
the top alternative of a non-streaming result or, of a
streaming result where is_final=true. This field is not
guaranteed to be accurate and users should not rely on it to
be always provided. The default of 0.0 is a sentinel value
indicating confidence was not set.