Define the metaphor and explain the two-part comparison in Lady Macbeth's line:"The raven himself is hoarse/That croaks the fatal entrance of...

In Shakespeare's Macbeth, Act 1.5,
the raven, a bird of evil portent and usually associated with death, is hoarse from
calling out the arrival of Duncan.  The idea is that the raven, a harbinger of death, is
so excited that it has gone hoarse from proclaiming Duncan's arrival so vigorously. 
Someone, or something, goes hoarse from talking too much, not from talking too
little. 

The "fatal" in the line has two meanings:  fatal
for Duncan, of course, but fatal also in the sense that it is fate or destiny that
Duncan will die. 

In short, Lady Macbeth performs what we,
today, call a pathetic fallacy.  She projects her own excitement on to the raven, seeing
the raven as just as excited by the idea of Duncan's death as she
is. 

The passage, then, certainly is anthropomorphic.  Lady
Macbeth attributes human behavior to a bird.   

But, unless
birds are incapable of becoming hoarse, the lines do not constitute a metaphor.  If
birds can't become hoarse, then the raven is being compared to a person that yells too
loudly for too long, and therefore becomes hoarse.  Specifically, the bird is being
compared to the person who would announce Duncan's arrival at the castle.  But if birds
can become hoarse, then the lines aren't metaphorical. 

I
don't know anything about bird anatomy, so I can't definitely say one way or another,
but the lines don't appear to be metaphorical.  They are anthropomorphic, but not
metaphorical.