Bump go.uber.org/zap from 1.21.0 to 1.26.0 in /processor/spanmetricsprocessor #1414

Job	Run time
setup-environment	14m 9s
stability-test (TestStabilityMetricsOTLP)	54s
stability-test (TestStabilityMetricsOpenCensus)	51s
stability-test (TestStabilityMetricsCarbon)	57s
stability-test (TestStabilityMetricsSignalFx)	1m 2s
stability-test (TestStabilityTracesOpenCensus)	50s
stability-test (TestStabilityTracesSAPM)	1m 27s
stability-test (TestStabilityTracesOTLP)	1m 8s
stability-test (TestStabilityTracesJaegerGRPC)	1m 17s
stability-test (TestStabilityTracesZipkin)	1m 0s
	23m 35s

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