add Dataflow scopes to credential for StreamingBenchmark

[clairemcginty]

in light of CircleCI failing to setup a Dataflow client in the new streaming metrics:

[error] (run-main-0) com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request
[error] {
[error]   "error" : "invalid_scope",
[error]   "error_description" : "Empty or missing scope not allowed."
[error] }
[error] com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request
[error] {
[error]   "error" : "invalid_scope",
[error]   "error_description" : "Empty or missing scope not allowed."
[error] }
[error] 	at com.google.api.client.auth.oauth2.TokenResponseException.from(TokenResponseException.java:105)
[error] 	at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:287)
[error] 	at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:307)
[error] 	at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:394)
[error] 	at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:489)
[error] 	at com.google.api.client.auth.oauth2.Credential.intercept(Credential.java:217)
[error] 	at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:868)
[error] 	at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:419)
[error] 	at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:352)
[error] 	at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:469)
[error] 	at com.spotify.ScioStreamingBenchmarkMetrics$.main(ScioStreamingBenchmark.scala:129)
[error] 	at com.spotify.ScioStreamingBenchmarkMetrics.main(ScioStreamingBenchmark.scala)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] 	at java.lang.reflect.Method.invoke(Method.java:498)

[codecov-io]

Codecov Report

Merging #1338 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1338   +/-   ##
=======================================
  Coverage   83.03%   83.03%           
=======================================
  Files         125      125           
  Lines        4037     4037           
  Branches      367      445   +78     
=======================================
  Hits         3352     3352           
  Misses        685      685

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