The two comments are not really conflicted.
You need to take into account though that both idiom and metaphor have a more strict and more relaxed definitions.
- a group of words established by usage as having a meaning not deducible from those of the individual words (e.g. see the light).
- a form of expression natural to a language, person, or group of people.
- a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable
- a thing regarded as representative or symbolic of something else
For metaphor wikipedia states that in broader sense "antithesis, hyperbole, metonymy and simile would all be considered types of metaphor".
Built on top of the basic terms the comments talk about
- metaphors that become idiomatic
- idiomatic metaphors
Metaphors that become idiomatic
One good example here is "silver lining", the metaphor is from a poem and is intuitively understandable. It is also a nice phrase and it took hold in the language. The expression through use acquired and fixed its meaning and now it is an idiom.
A different metaphor that become idiomatic is chip. Originally chip was O.E. cipp and meant only "piece of wood,". Since 1769 it is used for "thin slices of foodstuffs (originally fruit)."
However, this did not happen over night. When new meanings of the words develop the words are necessarily used1 metaphorically (most slang is metaphorical in some way). What ended up idiomatic in this development is that this meaning was kept in BrE and not in AmE and this sense of chip became idiomatic for BrE. (The other meaning of "counter used in a game of chance", first recorded 1840 is idiomatic in AmE)
For example if "chip" in BrE sense was used in a poem as part of another metaphor, let's imagine a line "last of my last chips I would share with you", here there is metaphor at play, but understanding of it depends on the audience, hence you can call it idiomatic metaphor according to the 2nd meaning of idiomatic ("a form of expression natural to a language..."). Another example could be "my bucket's kicker" as metaphor for "my killer"; here an idiom is used to create a metaphor.
Also, this could mean a metaphor which is well known - for example time is money; here the metaphor has became so well known that the expression is set in its form and acquired proverbial status.
1 Used here means both "said as" and "understood as".