Is this a metaphor?

It looks like a metaphor describing either the short summer season or the brief lifetime of an individual. I don't know the context.

Yes. Its a metaphor beccause 'Sommer' is personified as a 'tenant' leasing part of the life time of the person to whom the sonnet is addressed.
It does not refer to the whole life time of the individual, but to that (early) part of their life in which 'beauty' can be attributed.

thank you!

Thank you!