Report logs in dev to server on iOS

Summary:
This diff adds server side logging for all log levels in development (some levels sampled)

There are two new mobile analytic event pipelines (that pipe to scuba table)

- rn_dev_logs ([pigeon](https://our.intern.facebook.com/intern/marauder/event/?name=rn_dev_logs) [scuba](https://fburl.com/scuba/pqxl6mf5))
- rn_dev_logs_sampled ([pigeon](https://our.intern.facebook.com/intern/marauder/event/?name=rn_dev_logs_sampled) [scuba](https://fburl.com/scuba/oqz6b5x3))

Notes:
- All React Native logs from JS and Native on iOS go through this path to be printed to the console, so everything will be caputured even though native errors/warnings do not necessarily show a red/yellow box
- All errors and warnings are logged
- Log level info is sampled

Reviewed By: sammy-SC

Differential Revision: D17789494

fbshipit-source-id: dea6359237dbd91f267949f5185a0c79bb4083b8

React/Base/RCTLog.h

@@ -78,6 +78,16 @@ RCT_EXTERN NSString *RCTFormatLog(
   NSString *message
 );
 
+/**
+ * A method to generate a string RCTLogLevel
+ */
+RCT_EXTERN NSString *RCTFormatLogLevel(RCTLogLevel);
+
+/**
+ * A method to generate a string from a RCTLogSource
+ */
+RCT_EXTERN NSString *RCTFormatLogSource(RCTLogSource);
+
 /**
  * The default logging function used by RCTLogXX.
  */

React/Base/RCTLog.mm

@@ -162,6 +162,25 @@ NSString *RCTFormatLog(
   return log;
 }
 
+NSString *RCTFormatLogLevel(RCTLogLevel level)
+{
+    NSDictionary *levelsToString = @{@(RCTLogLevelTrace) : @"trace",
+                                    @(RCTLogLevelInfo)    : @"info",
+                                    @(RCTLogLevelWarning) : @"warning",
+                                    @(RCTLogLevelFatal)   : @"fatal",
+                                    @(RCTLogLevelError)   : @"error"};
+    
+    return levelsToString[@(level)];
+}
+
+NSString *RCTFormatLogSource(RCTLogSource source)
+{
+    NSDictionary *sourcesToString = @{@(RCTLogSourceNative) : @"native",
+                                     @(RCTLogSourceJavaScript)    : @"js"};
+    
+    return sourcesToString[@(source)];
+}
+
 static NSRegularExpression *nativeStackFrameRegex()
 {
   static dispatch_once_t onceToken;