Prometheus example with ddsketch aggregator panics

[clippit]

In exmplae/prometheus/main.go, modify aggregator selector from Exact to Sketch

- 	selector := simple.NewWithExactMeasure()
+ 	selector := simple.NewWithSketchMeasure(ddsketch.NewDefaultConfig())

Then the whole example will panic with following stacktrace:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x13b8326]

goroutine 9 [running]:
github.com/DataDog/sketches-go/ddsketch.(*DDSketch).Merge(0x0, 0xc00009d740)
        /Users/zhangletian/Dev/go/pkg/mod/github.com/!data!dog/sketches-go@v0.0.0-20190923095040-43f19ad77ff7/ddsketch/ddsketch.go:92 +0x36
go.opentelemetry.io/otel/sdk/metric/aggregator/ddsketch.(*Aggregator).Merge(0xc0001002a0, 0x1545e40, 0xc00009d770, 0xc00007a780, 0xc000126000, 0x2)
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/aggregator/ddsketch/ddsketch.go:131 +0x122
go.opentelemetry.io/otel/sdk/metric/batcher/defaultkeys.(*Batcher).Process(0xc0000305c0, 0x1547240, 0xc000028078, 0xc00007a780, 0xc00009d620, 0x1, 0x1, 0xc000028530, 0x10, 0x153ea80, ...)
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/batcher/defaultkeys/defaultkeys.go:131 +0x70c
go.opentelemetry.io/otel/sdk/metric.(*SDK).checkpoint(0xc0000af180, 0x1547240, 0xc000028078, 0xc0000ac6e0, 0x1)
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/sdk.go:429 +0x257
go.opentelemetry.io/otel/sdk/metric.(*SDK).Collect(0xc0000af180, 0x1547240, 0xc000028078, 0x0)
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/sdk.go:393 +0x194
go.opentelemetry.io/otel/sdk/metric/controller/push.(*Controller).collect(0xc0000f0100, 0x1547240, 0xc000028078)
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/controller/push/push.go:181 +0x91
go.opentelemetry.io/otel/sdk/metric/controller/push.(*Controller).tick(0xc0000f0100)
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/controller/push/push.go:164 +0x4f
go.opentelemetry.io/otel/sdk/metric/controller/push.(*Controller).run(0xc0000f0100, 0xc000092120)
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/controller/push/push.go:155 +0x42
created by go.opentelemetry.io/otel/sdk/metric/controller/push.(*Controller).Start
        /Users/zhangletian/Dev/go/src/go.opentelemetry.io/otel/sdk/metric/controller/push/push.go:127 +0xf7