cmake CMT_HAVE_FLUENT_OTEL_PROTO examples

fluent/fluent-bit lib/cmetrics/CMakeLists.txt :189 

check_c_source_compiles("
  #include <fluent-otel-proto/fluent-otel_found.h>
  int main() {
     return fluent_otel_found();
  }" CMT_HAVE_FLUENT_OTEL_PROTO)