add Reset function to MemoryWriter (#99)

Since the MemoryWriter was updated to address race conditions, access to the
slice is now private, so it cannot be replaced with an empty slice when you
want to reset it. This is done in the spectator-go-runtime-metrics library,
and may be done in other places.

This change adds a Reset function to the MemoryWriter, which can be used to
reset the storage to an empty slice.

spectator/writer/writer.go

@@ -32,6 +32,12 @@ func (m *MemoryWriter) Lines() []string {
 	return slices.Clone(m.lines)
 }
 
+func (m *MemoryWriter) Reset() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.lines = []string{}
+}
+
 func (m *MemoryWriter) Close() error {
 	return nil
 }

spectator/writer/writer_test.go

@@ -92,3 +92,31 @@ func TestMemoryWriter_Write(t *testing.T) {
 		t.Errorf("expected 10000 lines written to writer but found %d", linesWritten)
 	}
 }
+
+func TestMemoryWriter_Reset(t *testing.T) {
+	w, err := NewWriter("memory", logger.NewDefaultLogger())
+	if err != nil {
+		t.Errorf("failed to create writer: %s", err)
+	}
+	mw := w.(*MemoryWriter)
+
+	linesLength := len(mw.Lines())
+	if linesLength != 0 {
+		t.Errorf("expected 0 lines written to writer but found %d", linesLength)
+	}
+
+	mw.Write("")
+	mw.Write("")
+
+	linesLength = len(mw.Lines())
+	if linesLength != 2 {
+		t.Errorf("expected 2 lines written to writer but found %d", linesLength)
+	}
+
+	mw.Reset()
+
+	linesLength = len(mw.Lines())
+	if linesLength != 0 {
+		t.Errorf("expected 0 lines written to writer but found %d", linesLength)
+	}
+}