[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/level_string.go

@@ -0,0 +1,28 @@
+// Code generated by "stringer -type=Level -trimprefix=Level"; 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[LevelAlways-0]
+	_ = x[LevelCritical-1]
+	_ = x[LevelError-2]
+	_ = x[LevelWarning-3]
+	_ = x[LevelInfo-4]
+	_ = x[LevelVerbose-5]
+}
+
+const _Level_name = "AlwaysCriticalErrorWarningInfoVerbose"
+
+var _Level_index = [...]uint8{0, 6, 14, 19, 26, 30, 37}
+
+func (i Level) String() string {
+	if i >= Level(len(_Level_index)-1) {
+		return "Level(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _Level_name[_Level_index[i]:_Level_index[i+1]]
+}