Prometheus: Reduce allocations parsing exemplars (#58959)

* Prometheus: Reduce allocations parsing exemplars

This reduces allocations for parsing exemplars.

name            old time/op    new time/op    delta
ExemplarJson-8    24.7ms ±15%    17.6ms ± 2%  -28.72%  (p=0.008 n=5+5)

name            old alloc/op   new alloc/op   delta
ExemplarJson-8    4.70MB ± 0%    3.58MB ± 0%  -23.79%  (p=0.008 n=5+5)

name            old allocs/op  new allocs/op  delta
ExemplarJson-8     72.6k ± 0%     69.1k ± 0%   -4.81%  (p=0.008 n=5+5)

* Ensure pairs is reset

pkg/tsdb/prometheus/querydata/framing_bench_test.go

@@ -37,6 +37,7 @@ func BenchmarkExemplarJson(b *testing.B) {
 	tCtx, err := setup(true)
 	require.NoError(b, err)
 	b.ResetTimer()
+	b.ReportAllocs()
 	for n := 0; n < b.N; n++ {
 		res := http.Response{
 			StatusCode: 200,