[link-metrics] define StatusSubTlv to simplify appending/parsing it (#8051)

Status sub-TLV has a single `uint8_t` status value so we can use
declare it as `UintTlvInfo<SubTlv::kStatus, uint8_t>` and then use
helper `Append<StatusSubTlv>` and `Read<StatusSubTlv>` to append
and read it.

src/core/thread/link_metrics.cpp

@@ -270,11 +270,11 @@ Error LinkMetrics::AppendReport(Message &aMessage, const Message &aRequestMessag
 
         if (seriesInfo == nullptr)
         {
-            SuccessOrExit(error = AppendStatusSubTlvToMessage(aMessage, kStatusSeriesIdNotRecognized));
+            SuccessOrExit(error = Tlv::Append<StatusSubTlv>(aMessage, kStatusSeriesIdNotRecognized));
         }
         else if (seriesInfo->GetPduCount() == 0)
         {
-            SuccessOrExit(error = AppendStatusSubTlvToMessage(aMessage, kStatusNoMatchingFramesReceived));
+            SuccessOrExit(error = Tlv::Append<StatusSubTlv>(aMessage, kStatusNoMatchingFramesReceived));
         }
         else
         {
@@ -373,27 +373,28 @@ exit:
 Error LinkMetrics::HandleManagementResponse(const Message &aMessage, const Ip6::Address &aAddress)
 {
     Error    error = kErrorNone;
-    Tlv      tlv;
     uint16_t offset;
+    uint16_t endOffset;
     uint16_t length;
-    uint16_t index = 0;
-    Status   status;
+    uint8_t  status;
     bool     hasStatus = false;
 
     VerifyOrExit(mMgmtResponseCallback != nullptr);
 
     SuccessOrExit(error = Tlv::FindTlvValueOffset(aMessage, Mle::Tlv::Type::kLinkMetricsManagement, offset, length));
+    endOffset = offset + length;
 
-    while (index < length)
+    while (offset < endOffset)
     {
-        SuccessOrExit(aMessage.Read(offset + index, tlv));
+        Tlv tlv;
+
+        SuccessOrExit(error = aMessage.Read(offset, tlv));
 
         switch (tlv.GetType())
         {
-        case SubTlv::kStatus:
+        case StatusSubTlv::kType:
             VerifyOrExit(!hasStatus, error = kErrorParse);
-            VerifyOrExit(tlv.GetLength() == sizeof(status), error = kErrorParse);
-            SuccessOrExit(aMessage.Read(offset + index + sizeof(tlv), status));
+            SuccessOrExit(error = Tlv::Read<StatusSubTlv>(aMessage, offset, status));
             hasStatus = true;
             break;
 
@@ -401,7 +402,7 @@ Error LinkMetrics::HandleManagementResponse(const Message &aMessage, const Ip6::
             break;
         }
 
-        index += static_cast<uint16_t>(tlv.GetSize());
+        offset += sizeof(Tlv) + tlv.GetLength();
     }
 
     VerifyOrExit(hasStatus, error = kErrorParse);
@@ -813,20 +814,6 @@ exit:
     return error;
 }
 
-Error LinkMetrics::AppendStatusSubTlvToMessage(Message &aMessage, Status aStatus)
-{
-    Error error = kErrorNone;
-    Tlv   statusTlv;
-
-    statusTlv.SetType(SubTlv::kStatus);
-    statusTlv.SetLength(sizeof(uint8_t));
-    SuccessOrExit(error = aMessage.AppendBytes(&statusTlv, sizeof(statusTlv)));
-    SuccessOrExit(error = aMessage.AppendBytes(&aStatus, sizeof(aStatus)));
-
-exit:
-    return error;
-}
-
 } // namespace LinkMetrics
 } // namespace ot
 

src/core/thread/link_metrics.hpp

@@ -291,7 +291,6 @@ private:
                                             uint16_t       aEndPos,
                                             Metrics &      aMetrics);
     static Error AppendReportSubTlvToMessage(Message &aMessage, const MetricsValues &aValues);
-    static Error AppendStatusSubTlvToMessage(Message &aMessage, Status aStatus);
 
     ReportCallback                mReportCallback;
     void *                        mReportCallbackContext;

src/core/thread/link_metrics_tlvs.hpp

@@ -80,6 +80,12 @@ public:
  */
 typedef UintTlvInfo<SubTlv::kQueryId, uint8_t> QueryIdSubTlv;
 
+/**
+ * This type defines a Link Metrics Status Sub-Tlv.
+ *
+ */
+typedef UintTlvInfo<SubTlv::kStatus, uint8_t> StatusSubTlv;
+
 /**
  * This class implements Link Metrics Report Sub-TLV generation and parsing.
  *