[telemetry] border routing packet counters (#8280)

This commit introduces `otBorderRoutingCounters` which contains the
number of packets and bytes forwarded via border routing.

The counters have 4 categories: inbound unicast, inbound multicast,
outbound unicast, outbound multicast.

I've verified this feature locally on a BR device by ping command in
following scenarios:
- A host pings an ED's OMR.
- An ED pings a host's onlink address.
- A host pings a multicast address which an ED subscribes to.
- An ED pings a multicast address which a host subscribes to.

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (259)
+#define OPENTHREAD_API_VERSION (260)
 
 /**
  * @addtogroup api-instance

include/openthread/ip6.h

@@ -851,6 +851,40 @@ otError otIp6SetMeshLocalIid(otInstance *aInstance, const otIp6InterfaceIdentifi
  */
 const char *otIp6ProtoToString(uint8_t aIpProto);
 
+/**
+ * This structure represents the counters for packets and bytes.
+ *
+ */
+typedef struct otPacketsAndBytes
+{
+    uint64_t mPackets; ///< The number of packets.
+    uint64_t mBytes;   ///< The number of bytes.
+} otPacketsAndBytes;
+
+/**
+ * This structure represents the counters of packets forwarded via Border Routing.
+ *
+ */
+typedef struct otBorderRoutingCounters
+{
+    otPacketsAndBytes mInboundUnicast;    ///< The counters for inbound unicast.
+    otPacketsAndBytes mInboundMulticast;  ///< The counters for inbound multicast.
+    otPacketsAndBytes mOutboundUnicast;   ///< The counters for outbound unicast.
+    otPacketsAndBytes mOutboundMulticast; ///< The counters for outbound multicast.
+} otBorderRoutingCounters;
+
+/**
+ * Gets the Border Routing counters.
+ *
+ * This function requires the build-time feature `OPENTHREAD_CONFIG_IP6_BR_COUNTERS_ENABLE` to be enabled.
+ *
+ * @param[in]  aInstance  A pointer to an OpenThread instance.
+ *
+ * @returns A pointer to the Border Routing counters.
+ *
+ */
+const otBorderRoutingCounters *otIp6GetBorderRoutingCounters(otInstance *aInstance);
+
 /**
  * @}
  *