add support for Logger.with_context(**context) {} for per-scope message context

Author: HoneyryderChuck

lib/logger.rb

@@ -418,6 +418,17 @@ def with_level(severity)
     end
   end
 
+  def with_context(context)
+    prev_context = @context
+    @context = @context.nil? ? context : @context.merge(context)
+
+    begin
+      yield
+    ensure
+      @context = prev_context
+    end
+  end
+
   # Program name to include in log messages.
   attr_accessor :progname
 
@@ -605,7 +616,7 @@ def initialize(logdev, shift_age = 0, shift_size = 1048576, level: DEBUG,
     @default_formatter = Formatter.new
     self.datetime_format = datetime_format
     self.formatter = formatter
-    @logdev = nil
+    @logdev = @context = nil
     @level_override = {}
     return unless logdev
     case logdev
@@ -797,6 +808,15 @@ def level_key
   end
 
   def format_message(severity, datetime, progname, msg, **kwargs)
-    (@formatter || @default_formatter).call(severity, datetime, progname, msg, **kwargs)
+    context = @context
+    unless kwargs.empty?
+      context = context.nil? ? kwargs : context.merge(kwargs)
+    end
+
+    if context.nil?
+      (@formatter || @default_formatter).call(severity, datetime, progname, msg)
+    else
+      (@formatter || @default_formatter).call(severity, datetime, progname, msg, **context)
+    end
   end
 end