[XLA] Show metric name in categories table header.

Instead of

   ********** microseconds above estimated optimum report **********
   [...]

   ********** categories table **********
   The left hand side numbers are microseconds above estimated optimum.
   [...]

we now print

   ********** microseconds above estimated optimum report **********
   [...]
   ********** categories table for microseconds above estimated optimum **********
   [...]

which I think is more explicit and harder to misread.

PiperOrigin-RevId: 207325046