util/CircularBuffer: add method GetSize()

src/util/CircularBuffer.hxx

@@ -98,6 +98,15 @@ public:
 		return Next(tail) == head;
 	}
 
+	/**
+	 * Returns the number of elements stored in this buffer.
+	 */
+	constexpr size_type GetSize() const {
+		return head <= tail
+			? tail - head
+			: capacity - head + tail;
+	}
+
 	/**
 	 * Prepares writing.  Returns a buffer range which may be written.
 	 * When you are finished, call Append().