There actually really are not many metaphors portraying the message that love is eternal. On the contrary, there are actually far more relaying the message that love, especially violent, passionate love is very fleeting.
One metaphor that can be found that speaks of the eternity of love is spoken by Juliet in the famous balcony scene, Act 2, Scene 2. Speaking to Romeo, she compares the depths of her love to the depths of the sea, calling both eternally deep, or "infinite." Since the ocean is infinite, she is also saying that her love will last infinitely long, as we see in her lines:
My bounty is as boundless as the sea,
My love as deep; the more I give to thee,
The more I have, for both are infinite. (II.ii.139-141)
However, Juliet's metaphor is a flawed metaphor describing both the sea and love. We all know that the sea actually is not bottomless; therefore, Juliet's love and love in general is also not bottomless.
The truer metaphors are the ones showing us just how fleeting love can be. One metaphor describing the brevity of love is spoken by Juliet herself in this very same scene. She argues that Romeo should not proclaim his faithful love for her at this precise moment because their love is "too rash, too unadvis'd, too sudden" (124). She further argues that their love is not likely to last and compares it to a bolt of lightening, "Too like the lightning, which doth cease to be / Ere one can say 'It lightens.'" (125). Since bolts of lightning truly are short lived, this is a much truer metaphor than her infinite sea metaphor.
Even Friar Laurence speaks a metaphor that describes the impermanence of love just before he marries the couple. Friar Laurence warns Romeo that his love is not likely to last. In an earlier scene, he even argued that Romeo is far too young to understand what love really is. Instead, Friar Laurence asserts that Romeo might soon discover that such violent passion usually dies a violent death and compares passion to a fire being put out by powder, as we see in his lines:
These violent delights have violent ends
And in their triumph die, like fire and powder,
Which, as they kiss, consume. (II.vi.9-11)
Therefore, all in all, Shakespeare is not really showing us that love is eternal; rather, he is showing us that love, especially love guided by violent, uncontrolled passions is not genuine love and very fleeting.