[dsmr] Add internal 'telegram' text_sensor to support bridging (#3890)

Co-authored-by: H. Árkosi Róbert <robreg@zsurob.hu>
Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

components/sensor/dsmr.rst

@@ -256,6 +256,11 @@ Configuration variables:
 
   - All options from :ref:`Text Sensor <config-text_sensor>`.
 
+- **telegram** (*Optional*): The (decrypted) unparsed telegram, marked as internal sensor. 
+  Can also be used to trigger an action based on the last values. 
+
+  - All other options from :ref:`Text Sensor <config-text_sensor>`.
+
 Belgium
 
 - **p1_version_be** (*Optional*): DSMR Version Belgium
@@ -384,6 +389,42 @@ actually make it work, serial logging must be disabled to keep the hardware UART
       pin: GPIO13
       baud_rate: 115200
 
+Bridging support / raw telegram logging
+---------------------------------------
+
+You can use another uart to supply another P1 receiver with the same telegram. See configuration sample as used for bridging.
+
+.. code-block:: yaml
+
+    # define multiple uart's
+    uart:
+      - id: p1_uart
+        rx_pin:
+          number: 4
+          inverted: true
+        baud_rate: 115200
+        rx_buffer_size: 1700
+      - id: p1_bridge_uart
+        tx_pin:
+          number: 10
+        baud_rate: 115200
+
+    # link input uart to dsmr
+    dsmr:
+      uart_id: p1_uart
+      max_telegram_length: 1700
+
+    # log the telegram and pass telegram to p1_bridge_uart 
+    text_sensor:
+      - platform: dsmr
+        telegram:
+          name: "telegram"
+          on_value:
+            then:
+              - lambda: |-
+                  ESP_LOGV("dsrm", "telegram: %s", x.c_str());
+                  p1_bridge_uart->write_str(x.c_str());
+
 See Also
 --------