Register Stackdriver auto-config in META-INF/spring.factories

See gh-19955

Author: wonwoo

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/resources/META-INF/spring.factories

@@ -62,6 +62,7 @@ org.springframework.boot.actuate.autoconfigure.metrics.export.newrelic.NewRelicM
 org.springframework.boot.actuate.autoconfigure.metrics.export.prometheus.PrometheusMetricsExportAutoConfiguration,\
 org.springframework.boot.actuate.autoconfigure.metrics.export.signalfx.SignalFxMetricsExportAutoConfiguration,\
 org.springframework.boot.actuate.autoconfigure.metrics.export.simple.SimpleMetricsExportAutoConfiguration,\
+org.springframework.boot.actuate.autoconfigure.metrics.export.stackdriver.StackdriverMetricsExportAutoConfiguration,\
 org.springframework.boot.actuate.autoconfigure.metrics.export.statsd.StatsdMetricsExportAutoConfiguration,\
 org.springframework.boot.actuate.autoconfigure.metrics.export.wavefront.WavefrontMetricsExportAutoConfiguration,\
 org.springframework.boot.actuate.autoconfigure.metrics.jdbc.DataSourcePoolMetricsAutoConfiguration,\