Change exemplars to slice of values instead of ptrs (open-telemetry#2402

)

Benchmarks before:
```
goos: darwin
goarch: amd64
pkg: go.opentelemetry.io/collector/consumer/pdata
BenchmarkMetricsFromOtlp
BenchmarkMetricsFromOtlp-16    	     322	   3809358 ns/op	 2271080 B/op	   51507 allocs/op
PASS
```

Benchmarks after
```
goos: darwin
goarch: amd64
pkg: go.opentelemetry.io/collector/consumer/pdata
BenchmarkMetricsFromOtlp
BenchmarkMetricsFromOtlp-16    	     331	   3594315 ns/op	 3212271 B/op	   34700 allocs/op
PASS
```

Significant improvement in the number of allocations. Increase in the number of total bytes allocated.

Signed-off-by: Bogdan Drutu <[email protected]>

metric.go

@@ -442,7 +442,7 @@ func generateOtlpDoubleHistogramMetric() *otlpmetrics.Metric {
 				Sum:               15,
 				BucketCounts:      []uint64{0, 1},
 				ExplicitBounds:    []float64{1},
-				Exemplars: []*otlpmetrics.DoubleExemplar{
+				Exemplars: []otlpmetrics.DoubleExemplar{
 					{
 						FilteredLabels: generateOtlpMetricAttachment(),
 						TimeUnixNano:   uint64(TestMetricExemplarTimestamp),
@@ -500,7 +500,7 @@ func generateOtlpIntHistogramMetric() *otlpmetrics.Metric {
 				Sum:               15,
 				BucketCounts:      []uint64{0, 1},
 				ExplicitBounds:    []float64{1},
-				Exemplars: []*otlpmetrics.IntExemplar{
+				Exemplars: []otlpmetrics.IntExemplar{
 					{
 						FilteredLabels: generateOtlpMetricAttachment(),
 						TimeUnixNano:   uint64(TestMetricExemplarTimestamp),