There are several metaphors in "Full Moon and Little Frieda."
"A spider's web, tense for the dew's touch" describes a spider's web that is seen at night (the setting of the poem) and how it looks before the dawn. Over the course of the evening, dew droplets from condensation in the air will gather on the veins of the web, to be seen sparkling in the early morning sun.
"A dark river of blood, many boulders..." refers to the movement of the water, which appears as blood in the dark.
In terms of the metaphor, it may figuratively describe how difficult it is to navigate, in general, through the dark (figuratively or literally), and that water takes the "path of least resistance," not fighting boulders or obstacles in the way, but moving around them. Perhaps the author suggests to the reader that this is the best course to take in our daily trials and tribulations. (The rest of the line, "balancing unspilled milk" may simply refer to staying focused on the task at hand as you move through the darkness).
"The moon has stepped back" is personification (which is a form of metaphor, comparing the moon to a person). In this case, the moon moves a step backward to gain a better vantage point as it gazes at the world below. In some ways, the description of the moon places what is on the earth and what is above the earth, on equal footing. (I am reminded of the poem "Desiderata," by Max Ehrmann which states "You are a child of the universe / no less than the trees and the stars...")
[The rest of the quote ("The moon has stepped back like an artist gazing amazed at a work / That points at him amazed") is a simile, which continues to lift up the idea of the heavens looking at us while we look at the heavens.]
"A pail lifted, still and brimming - mirror / To tempt a first star..." is a metaphor, comparing the surface of the brimming pail to that of a mirror. This image also reinforces the beauty of what is on the Earth and what is beautiful in the heavens, and that neither is more or less awe-inspiring. Hughes uses personification (as said before, a kind of imagery/metaphor) to remind us of the majesty of the creation of man (as seen in little Frieda), even as we admire the majesty of that above us, i.e., moon, stars, etc.
Ted Hughes uses imagery to describe, with substantive and lovely details, the scene of this particular night when Frieda notices, with sheer delight and awe, the moon above her.
The imagery also is "painted" in such a way that it seems to reflect the sense of that simile noted above: "The moon has stepped back like an artist gazing amazed at a work." The moon is compared to an artist who stares amazed at the beauty it observes beneath it with its reflected light—while that beauty (little Frieda), with equal amazement, points back at the moon.
Hope this is of some assistance to you.