[pdata] Deprecate HexString func on SpanID and TraceID

`[SpanID|TraceID].HexString` methods currently return an empty string for empty SpanID and TraceID instances. This behavior is not defined in the OTel spec and contradicts with w3c recommendations. We don't want to promote a behavior which possibly can be changed in future.

Additionally we introduce `[SpanID|TraceID].String` methods that have the same logic as HexString and can be used in log statement as Stringer instance to avoid unnecessary allocation when log is not emitted. But the `String` method is not encouraged to be used as a replacement for HexString because due to the concerns as described above.

.chloggen/deprecate_hexstring.yaml

@@ -0,0 +1,4 @@
+change_type: 'deprecation'
+component: pdata
+note: Deprecate `pcommon.[Span|Trace]ID.HexString` methods. Call `hex.EncodeToString` explicitly instead.
+issues: [6514]

pdata/pcommon/spanid.go

@@ -29,7 +29,19 @@ func NewSpanIDEmpty() SpanID {
 	return emptySpanID
 }
 
-// HexString returns hex representation of the SpanID.
+// HexString returns string representation of the SpanID.
+//
+// Important: Don't rely on this method to get a string identifier of SpanID,
+// Use hex.EncodeToString explicitly instead.
+// This method meant to implement Stringer interface for display purposes only.
+func (ms SpanID) String() string {
+	if ms.IsEmpty() {
+		return ""
+	}
+	return hex.EncodeToString(ms[:])
+}
+
+// Deprecated: [0.65.0] Call hex.EncodeToString explicitly instead.
 func (ms SpanID) HexString() string {
 	if ms.IsEmpty() {
 		return ""

pdata/pcommon/spanid_test.go

@@ -40,6 +40,14 @@ func TestSpanIDHexString(t *testing.T) {
 	assert.Equal(t, "1223ad1223ad1223", sid.HexString())
 }
 
+func TestSpanIDString(t *testing.T) {
+	sid := SpanID([8]byte{})
+	assert.Equal(t, "", sid.String())
+
+	sid = SpanID([8]byte{0x12, 0x23, 0xAD, 0x12, 0x23, 0xAD, 0x12, 0x23})
+	assert.Equal(t, "1223ad1223ad1223", sid.String())
+}
+
 func TestSpanIDImmutable(t *testing.T) {
 	initialBytes := [8]byte{0x12, 0x23, 0xAD, 0x12, 0x23, 0xAD, 0x12, 0x23}
 	sid := SpanID(initialBytes)

pdata/pcommon/traceid.go

@@ -30,7 +30,19 @@ func NewTraceIDEmpty() TraceID {
 	return emptyTraceID
 }
 
-// HexString returns hex representation of the TraceID.
+// HexString returns string representation of the TraceID.
+//
+// Important: Don't rely on this method to get a string identifier of TraceID.
+// Use hex.EncodeToString explicitly instead.
+// This method meant to implement Stringer interface for display purposes only.
+func (ms TraceID) String() string {
+	if ms.IsEmpty() {
+		return ""
+	}
+	return hex.EncodeToString(ms[:])
+}
+
+// Deprecated: [0.65.0] Call hex.EncodeToString explicitly instead.
 func (ms TraceID) HexString() string {
 	if ms.IsEmpty() {
 		return ""

pdata/pcommon/traceid_test.go

@@ -41,6 +41,14 @@ func TestTraceIDHexString(t *testing.T) {
 	assert.Equal(t, "12345678123456781234567812345678", tid.HexString())
 }
 
+func TestTraceIDString(t *testing.T) {
+	tid := TraceID([16]byte{})
+	assert.Equal(t, "", tid.String())
+
+	tid = [16]byte{0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78}
+	assert.Equal(t, "12345678123456781234567812345678", tid.String())
+}
+
 func TestTraceIDImmutable(t *testing.T) {
 	initialBytes := [16]byte{0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78}
 	tid := TraceID(initialBytes)