avfilter/af_astats: add support for selecting measured statistics

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>

doc/filters.texi

@@ -2156,6 +2156,17 @@ For description what each key means read below.
 @item reset
 Set number of frame after which stats are going to be recalculated.
 Default is disabled.
+
+@item measure_perchannel
+Select the entries which need to be measured per channel. The metadata keys can
+be used as flags, default is @option{all} which measures everything.
+@option{none} disables all per channel measurement.
+
+@item measure_overall
+Select the entries which need to be measured overall. The metadata keys can
+be used as flags, default is @option{all} which measures everything.
+@option{none} disables all overall measurement.
+
 @end table
 
 A description of each shown parameter follows: