mod_stats_prometheus: make bucket sizes configurable

[Saarko]

Currently bucket sizes for time and data are hard coded. These values are tailored for Audio/Video use cases, however, TURN is not only used for these scenarios. Therefore, provide a configurational option to define them via the module section, e.g. like this:

mod_stats_prometheus:
  buckets_time: [10, 20, 30, 45, 60, 120, 300]
  buckets_size: [1024, 2048, 4096, 8192] 

eturnal/src/mod_stats_prometheus.erl

Lines 31 to 50 in a5f7a13

 -define(SIZE_BUCKETS, 
         [1024 * 4, 
          1024 * 32, 
          1024 * 256, 
          1024 * 1024, 
          1024 * 1024 * 4, 
          1024 * 1024 * 16, 
          1024 * 1024 * 64, 
          1024 * 1024 * 256, 
          1024 * 1024 * 1024]). 
 -define(TIME_BUCKETS, 
         [timer:minutes(1) div 1000, 
          timer:minutes(5) div 1000, 
          timer:minutes(15) div 1000, 
          timer:minutes(30) div 1000, 
          timer:hours(1) div 1000, 
          timer:hours(2) div 1000, 
          timer:hours(6) div 1000, 
          timer:hours(12) div 1000, 
          timer:hours(24) div 1000]).