add exemplar filter as environment variable #4

Job	Run time
opentelemetry-api	33s
opentelemetry-exporter-opencensus	45s
opentelemetry-exporter-otlp-combined	47s
opentelemetry-exporter-otlp-proto-common	47s
opentelemetry-exporter-otlp-proto-grpc	53s
opentelemetry-exporter-otlp-proto-http	51s
opentelemetry-exporter-prometheus	41s
opentelemetry-exporter-zipkin-combined	39s
opentelemetry-exporter-zipkin-json	39s
opentelemetry-exporter-zipkin-proto-http	50s
opentelemetry-getting-started	1m 3s
opentelemetry-opencensus-shim	41s
opentelemetry-opentracing-shim	44s
opentelemetry-propagator-b3	42s
opentelemetry-propagator-jaeger	48s
opentelemetry-proto	34s
opentelemetry-sdk	41s
opentelemetry-semantic-conventions	36s
opentelemetry-test-utils	40s
	13m 54s

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