introspection: new 'Stats' property in the bearer object

The new property is a dictionary which may include different parameters,
depending on what is actually supported by the underlying modem. For now,
just bytes RX/TX.

Note that this object will expose the stats *as reported by the modem*. These
values may differ from e.g. what is seen in the network interface stats.

introspection/org.freedesktop.ModemManager1.Bearer.xml

@@ -241,6 +241,37 @@
     -->
     <property name="Ip6Config" type="a{sv}" access="read" />
 
+    <!--
+        Stats:
+
+        If the modem supports it, this property will show statistics of the
+        ongoing connection.
+
+        When the connection is disconnected automatically or explicitly by the
+        user, the values in this property will show the last values cached.
+        The statistics are reset
+
+        The following items may appear in the list of statistics:
+        <variablelist>
+          <varlistentry><term><literal>"rx-bytes"</literal></term>
+            <listitem>
+              Number of bytes received without error, given as an unsigned 64-bit integer value (signature <literal>"t"</literal>).
+            </listitem>
+          </varlistentry>
+          <varlistentry><term><literal>"tx-bytes"</literal></term>
+            <listitem>
+              Number bytes transmitted without error, given as an unsigned 64-bit integer value (signature <literal>"t"</literal>).
+            </listitem>
+            </varlistentry>
+          <varlistentry><term><literal>"duration"</literal></term>
+            <listitem>
+              Duration of the connection, in seconds, given as an unsigned integer value (signature <literal>"u"</literal>).
+            </listitem>
+          </varlistentry>
+        </variablelist>
+    -->
+    <property name="Stats" type="a{sv}" access="read" />
+
     <!--
         IpTimeout: