A memory leak cause by the active set of spanStore

[NanoRed]

Please answer these questions before submitting a bug report.

What version of OpenCensus are you using?

v0.22.5

What version of Go are you using?

1.13.15

What did you do?

import (
	"go.opencensus.io/trace"
	_ "go.opencensus.io/zpages" // internal.LocalSpanStoreEnabled = true
)

type SpanTest struct {
	Ctx   context.Context
	Count int
}

func (s *SpanTest) Inc() {
	span := trace.StartSpan(s.Ctx, "test")
	defer span.End()
}

func main() {
	test := &SpanTest{
		Ctx: context.Background(),
	}
	go func() {
		for {
			test.Inc()
		}
	}()
	for {}
}

it make memory usage gradually increasing.
and i have found the problem:
opencensus-go/trace/trace.go line:269 ss.add(NewSpan(s))
opencensus-go/trace/trace.go line:294 s.spanStore.finished(NewSpan(s), sd)
when add a new span to the spanStore active set. it wrote a wrong way to release the active set, it shouldn't use NewSpan() to wrap the *span and pass to the finished function

[NanoRed]

has been fixed