What are the rhyme scheme and meter of the poem "Ozymandias"?

"Ozymandias" is written in iambic pentameter. This is a meter in which a line has five pairs of two syllables, for a total of ten syllables. In an iambic meter, the stress falls on the second syllable in the pair. For example, the opening line reads as follows:

I met a traveller from an antique land

The bolded words indicate where the stresses fall. Critics have noted that some of the lines in the sonnet don't appear to follow iambic pentameter precisely (unless you read them in an unnatural way), but in general, the poem has the "da-DUM da-DUM" meter of iambic pentameter—and we can only quibble as to where the stresses fall because we don't know how Shelley would have read the poem.
As for rhyme scheme, the first four lines follow the pattern of the Shakespearean sonnet: A-B-A-B, with "land" and "sand" and "stone" and "frown" rhyming: however, stone and frown don't exactly rhyme. This is called a slant rhyme. Some scholars argue that "stone" and "frown" rhymed more closely in Shelly's day, but again, we don't know, as there's no way to know how Shelley would have read it.
After this, the next quatrain or four line group rhymes A-C-D-C: "command" rhymes with "land" and "sand", and then "read" and "fed" rhyme. In the next quatrain, lines 9–12, the rhyme is E-D-E-F. Usually a sonnet ends on rhyming couplet, meaning the last two words rhyme with each other, but in this case, the rhyme is E-F: "bare" rhymes with "despair" and "away" with "decay." To me, this is very effective, because it slows down the end of the poem and gives it a hollow, desolate sound.
Despite some possible variants, this sonnet is written in a traditional iambic pentameter meter and with end rhymes that vary a bit from traditional sonnet patterns but still work effectively.