[FEATURE+BUG] Set the update interval of the stats file

[OA6DXV]

I know that in the configuration files, an update time of 15 seconds is defined for the file that exports statistics (to later use with Prometheus). I want to propose a modification to the code by declaring the integer variable stats_update_interval so that it can be added to the .conf file at the beginning, in the global variables section.

I already have the code where rtl_airband.h, rtl_airband.cpp, and output.cpp were modified, and it works very well. The problem is that I ran these tests after cloning the main branch. Now that I'm trying to prepare for the pull request using the unstable branch (as I was instructed in a previous PR), I can't get the compilation to work due to errors unrelated to the changes I made.

[  4%] Generating version.cpp, _version.cpp
[  9%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/config.cpp.o
[ 14%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/input-common.cpp.o
[ 19%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/input-file.cpp.o
[ 23%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/input-helpers.cpp.o
[ 28%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/mixer.cpp.o
[ 33%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/output.cpp.o
[ 38%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/rtl_airband.cpp.o
[ 42%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/squelch.cpp.o
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[ 52%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/util.cpp.o
[ 57%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/udp_stream.cpp.o
[ 61%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/logging.cpp.o
[ 66%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/filters.cpp.o
[ 71%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/helper_functions.cpp.o
[ 71%] Generating version.cpp, _version.cpp
[ 76%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/version.cpp.o
[ 80%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/input-rtlsdr.cpp.o
[ 85%] Building CXX object src/CMakeFiles/rtl_airband_base.dir/input-mirisdr.cpp.o
/RTLSDR-Airband/src/input-mirisdr.cpp: In function ‘int mirisdr_find_device_by_serial(const char*)’:
/RTLSDR-Airband/src/input-mirisdr.cpp:72:13: error: declaration of ‘device_count’ shadows a global declaration [-Werror=shadow]
   72 |         int device_count;
      |             ^~~~~~~~~~~~
In file included from /RTLSDR-Airband/src/input-mirisdr.cpp:34:
/RTLSDR-Airband/src/rtl_airband.h:345:12: note: shadowed declaration is here
  345 | extern int device_count, mixer_count;
      |            ^~~~~~~~~~~~
/RTLSDR-Airband/src/input-mirisdr.cpp: In function ‘int mirisdr_init(input_t*)’:
/RTLSDR-Airband/src/input-mirisdr.cpp:106:44: error: cast from type ‘const char*’ to type ‘char*’ casts away qualifiers [-Werror=cast-qual]
  106 |         int r = mirisdr_set_transfer(miri, (char *)"BULK");
      |                                            ^~~~~~~~~~~~~~
/RTLSDR-Airband/src/input-mirisdr.cpp:135:45: error: cast from type ‘const char*’ to type ‘char*’ casts away qualifiers [-Werror=cast-qual]
  135 |         r = mirisdr_set_sample_format(miri, (char *)"504_S8");
      |                                             ^~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [src/CMakeFiles/rtl_airband_base.dir/build.make:307: src/CMakeFiles/rtl_airband_base.dir/input-mirisdr.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:100: src/CMakeFiles/rtl_airband_base.dir/all] Error 2
make: *** [Makefile:136: all] Error 2