set_metadata with many entries is not very efficient, and with small audio
frames the performance loss is noticable. Also with this very simple
calculations (like peak) can be even further optimized.

Unfoturnately there are some small differences in metadata and av_log info
output, so factorizing calculations and output might not worth the hassle.
Signed-off-by: Marton Balint <cus@passwd.hu>