Fix enabling debug logging from Unity in iOS.

Previously, setting FirebaseFirestore.LogLevel to LogLevel.Debug would get bumped back to LogLevel.Info as an unexpected side effect of creating of a new firebase::App object. The apparent effect to users was that enabling debug logging had no effect. This made it challenging to debug issues with customers because they were unable to collect valuable debug logs when using iOS as their platform.

PiperOrigin-RevId: 319056991

Author: Googler

firestore/src/ios/firestore_ios.cc

@@ -270,21 +270,28 @@ void Firestore::set_log_level(LogLevel log_level) {
     case kLogLevelDebug:
       // Firestore doesn't have the distinction between "verbose" and "debug".
       LogSetLevel(util::kLogLevelDebug);
-      return;
+      break;
     case kLogLevelInfo:
       LogSetLevel(util::kLogLevelNotice);
-      return;
+      break;
     case kLogLevelWarning:
       LogSetLevel(util::kLogLevelWarning);
-      return;
+      break;
     case kLogLevelError:
     case kLogLevelAssert:
       // Firestore doesn't have a separate "assert" log level.
       LogSetLevel(util::kLogLevelError);
-      return;
+      break;
+    default:
+      UNREACHABLE();
+      break;
   }
 
-  UNREACHABLE();
+  // Call SetLogLevel() to keep the C++ log level in sync with FIRLogger's.
+  // Convert kLogLevelDebug to kLogLevelVerbose to force debug logs to be
+  // emitted. See b/159048318 for details.
+  firebase::SetLogLevel(log_level == kLogLevelDebug ? kLogLevelVerbose
+                                                    : log_level);
 }
 
 }  // namespace firestore