Show mean throughput in command line report

[danielmitterdorfer]

With this commit Rally shows also the mean throughput in the command
line report. The mean is useful e.g. for calculating summary statistics
across several races or for conducting statistical significance tests.

[DJRickyB]

since means will almost always be greater than median, I'd suggest either putting it above min (as its own "class") or between median and max. in fact I think I'd prefer the former (put it into its own)
Edit: actually I guess I don't know for sure which way the mean will go

[danielmitterdorfer]

since means will almost always be greater than median, I'd suggest either putting it above min (as its own "class") or between median and max. in fact I think I'd prefer the former (put it into its own)
Edit: actually I guess I don't know for sure which way the mean will go

Yes, the mean can either be smaller or larger than the median, depending on how the distribution is skewed. I am fine either way but I propose that we leave it as is, indicating at least that mean and median are in between min and max.

Also one integration test has failed. In the Elasticsearch log we see:

[2021-01-12T10:22:39,196][ERROR][o.e.b.Bootstrap          ] [rally-node-0] Exception
org.elasticsearch.http.BindHttpException: Failed to bind to [39200]
	at org.elasticsearch.http.netty4.Netty4HttpServerTransport.bindAddress(Netty4HttpServerTransport.java:487) ~[?:?]
	at org.elasticsearch.http.netty4.Netty4HttpServerTransport.createBoundHttpAddress(Netty4HttpServerTransport.java:384) ~[?:?]
	at org.elasticsearch.http.netty4.Netty4HttpServerTransport.doStart(Netty4HttpServerTransport.java:361) ~[?:?]
	at org.elasticsearch.common.component.AbstractLifecycleComponent.start(AbstractLifecycleComponent.java:72) ~[elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.node.Node.start(Node.java:809) ~[elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.bootstrap.Bootstrap.start(Bootstrap.java:269) ~[elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.bootstrap.Bootstrap.init(Bootstrap.java:342) [elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.bootstrap.Elasticsearch.init(Elasticsearch.java:159) [elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.bootstrap.Elasticsearch.execute(Elasticsearch.java:150) [elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:86) [elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.cli.Command.mainWithoutErrorHandling(Command.java:124) [elasticsearch-cli-6.8.0.jar:6.8.0]
	at org.elasticsearch.cli.Command.main(Command.java:90) [elasticsearch-cli-6.8.0.jar:6.8.0]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:116) [elasticsearch-6.8.0.jar:6.8.0]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:93) [elasticsearch-6.8.0.jar:6.8.0]
Caused by: java.net.BindException: Address already in use

i.e. this is unrelated to this PR and considered transient. @elasticmachine test this please

[DJRickyB]

LGTM