Update noop.go

[chanxuehong]

as SetTag, SetOperationName
i think there should be

return n

rather than

return defaultNoopSpan

Specify type to avoid type conversion when return these values as interface

	defaultNoopSpanContext SpanContext = noopSpanContext{}
	defaultNoopSpan        Span        = noopSpan{}
	defaultNoopTracer      Tracer      = NoopTracer{}

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@16ba2b6). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #223   +/-   ##
=========================================
  Coverage          ?   59.54%           
=========================================
  Files             ?       14           
  Lines             ?      833           
  Branches          ?        0           
=========================================
  Hits              ?      496           
  Misses            ?      297           
  Partials          ?       40

Impacted Files	Coverage Δ
noop.go	5% <0%> (ø)

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