One must begin to answer this question by defining both metaphor and symbol.
A metaphor, according to eNotes, is:
a figure of speech in which a word or phrase is applied to a person, idea, or object to which it is not literally applicable.
A symbol is an object or word/phrase which represents, or stands for, another object/word/phrase.
Therefore, depending upon how one looks at the idea of the catcher in the rye, one could justify as being either a metaphor or a symbol.
If regarding the catcher in the rye as something which Holden wishes to be one day, or which he compares himself to, then one could justify that the image exists as a metaphor.
In regarding to the symbolism,the catcher in the rye represents something for Holden. The catcher represents his desire and need to feel needed and protect the innocent. Here, it seems that he acknowledges his shortcomings as a youth and wishes to protect others from making the same mistakes he has made in his life. The catcher in the rye represents what Holden wishes to be. If looking at him through the eyes of Phoebe, one could certainly see him as a protector of innocence.