Separate ETW into low-level and high-level API

internal/etw/fieldopt.go

@@ -0,0 +1,35 @@
+package etw
+
+// FieldOpt defines the option function type that can be passed to
+// Provider.WriteEvent to add fields to the event.
+type FieldOpt func(em *EventMetadata, ed *EventData)
+
+// StringField adds a single string field to the event.
+func StringField(name string, value string) FieldOpt {
+	return func(em *EventMetadata, ed *EventData) {
+		em.WriteField(name, InTypeANSIString, OutTypeUTF8, 0)
+		ed.WriteString(value)
+	}
+}
+
+// StringArray adds an array of strings to the event.
+func StringArray(name string, values []string) FieldOpt {
+	return func(em *EventMetadata, ed *EventData) {
+		em.WriteArray(name, InTypeANSIString, OutTypeUTF8, 0)
+		ed.WriteUint16(uint16(len(values)))
+		for _, v := range values {
+			ed.WriteString(v)
+		}
+	}
+}
+
+// Struct adds a nested struct to the event, the FieldOpts in the opts argument
+// are used to specify the fields of the struct.
+func Struct(name string, opts ...FieldOpt) FieldOpt {
+	return func(em *EventMetadata, ed *EventData) {
+		em.WriteStruct(name, uint8(len(opts)), 0)
+		for _, opt := range opts {
+			opt(em, ed)
+		}
+	}
+}