[etw] Add String() functions, JSON field option (#285)

Add `String()` functions to `etw.Level` and `etw.Opcode` types.

Add `etw.JSONStringField()`, which denotes a string field containing
JSON data.

Fix doc comment casing.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

pkg/etw/opcode_string.go

@@ -0,0 +1,27 @@
+// Code generated by "stringer -type=Opcode -trimprefix=Opcode"; DO NOT EDIT.
+
+package etw
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[OpcodeInfo-0]
+	_ = x[OpcodeStart-1]
+	_ = x[OpcodeStop-2]
+	_ = x[OpcodeDCStart-3]
+	_ = x[OpcodeDCStop-4]
+}
+
+const _Opcode_name = "InfoStartStopDCStartDCStop"
+
+var _Opcode_index = [...]uint8{0, 4, 9, 13, 20, 26}
+
+func (i Opcode) String() string {
+	if i >= Opcode(len(_Opcode_index)-1) {
+		return "Opcode(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _Opcode_name[_Opcode_index[i]:_Opcode_index[i+1]]
+}