restore 3p exporter feature gates to alpha stage #4056

Job	Run time
create-test-ref	2s
validate-markdown	30s
changes	8s
get-test-cases	8s
build-windows	32m 0s
build	22m 59s
packaging-deb	30s
packaging-msi	2m 2s
packaging-rpm	55s
run-test-case (statsd_mock)	2m 16s
run-test-case (otlp_metric_mock)	2m 13s
run-test-case (otlp_mock)	2m 18s
run-test-case (otlp_grpc_exporter_metric_mock)	2m 49s
run-test-case (otlp_grpc_exporter_trace_mock)	3m 0s
run-test-case (otlp_http_exporter_metric_mock)	2m 15s
run-test-case (otlp_http_exporter_trace_mock)	2m 32s
run-test-case (sapm_exporter_trace_mock)	2m 32s
run-test-case (signalfx_exporter_metric_mock)	2m 24s
run-test-case (datadog_exporter_metric_mock)	1m 58s
run-test-case (datadog_exporter_trace_mock)	2m 3s
run-test-case (prometheus_mock)	2m 46s
run-test-case (zipkin_mock)	2m 24s
run-test-case (jaeger_mock)	2m 19s
run-test-case (logzio_exporter_trace_mock)	2m 17s
test-control-stript	12s
	1h 35m 32s

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