Correctly handle pruning wraparound after ring expansion (#3942)

Fixes #3939

storage/buffer.go

@@ -178,6 +178,7 @@ func (r *sampleRing) add(t int64, v float64) {
 		r.buf = buf
 		r.i = r.f
 		r.f += l
+		l = 2 * l
 	} else {
 		r.i++
 		if r.i >= l {

storage/buffer_test.go

@@ -48,6 +48,11 @@ func TestSampleRing(t *testing.T) {
 			delta: 7,
 			size:  1,
 		},
+		{
+			input: []int64{1, 2, 3, 4, 6},
+			delta: 4,
+			size:  4,
+		},
 	}
 	for _, c := range cases {
 		r := newSampleRing(c.delta, c.size)