[mesh-forwarder] add method ResetCounters to clear IP counter (#4286)

This commit also adds a public API `otThreadResetIp6Counters()`.

include/openthread/thread.h

@@ -730,6 +730,14 @@ otError otThreadSendDiagnosticReset(otInstance *        aInstance,
  */
 const otIpCounters *otThreadGetIp6Counters(otInstance *aInstance);
 
+/**
+ * Reset the IPv6 counters.
+ *
+ * @param[in]  aInstance  A pointer to an OpenThread instance.
+ *
+ */
+void otThreadResetIp6Counters(otInstance *aInstance);
+
 /**
  * Get the Thread MLE counters.
  *

src/core/api/thread_api.cpp

@@ -454,6 +454,13 @@ const otIpCounters *otThreadGetIp6Counters(otInstance *aInstance)
     return &instance.Get<MeshForwarder>().GetCounters();
 }
 
+void otThreadResetIp6Counters(otInstance *aInstance)
+{
+    Instance &instance = *static_cast<Instance *>(aInstance);
+
+    instance.Get<MeshForwarder>().ResetCounters();
+}
+
 const otMleCounters *otThreadGetMleCounters(otInstance *aInstance)
 {
     Instance &instance = *static_cast<Instance *>(aInstance);

src/core/thread/mesh_forwarder.cpp

@@ -78,10 +78,7 @@ MeshForwarder::MeshForwarder(Instance &aInstance)
 {
     mFragTag = Random::NonCrypto::GetUint16();
 
-    mIpCounters.mTxSuccess = 0;
-    mIpCounters.mRxSuccess = 0;
-    mIpCounters.mTxFailure = 0;
-    mIpCounters.mRxFailure = 0;
+    ResetCounters();
 
 #if OPENTHREAD_FTD
     memset(mFragmentEntries, 0, sizeof(mFragmentEntries));

src/core/thread/mesh_forwarder.hpp

@@ -297,6 +297,12 @@ public:
      */
     const otIpCounters &GetCounters(void) const { return mIpCounters; }
 
+    /**
+     * This method resets the IP level counters.
+     *
+     */
+    void ResetCounters(void) { memset(&mIpCounters, 0, sizeof(mIpCounters)); }
+
 #if OPENTHREAD_FTD
     /**
      * This method returns a reference to the resolving queue.