Only index spans if field mapping was created by stagemonitor

[trampi]

If the monitored app is started before elasticsearch is available, the stagemonitor span index pattern is not created, as the index pattern is only created on application startup, see the corresponding code in ElasticsearchTracingPlugin.java

A solution would be to assure the creation of the index pattern before signalising that elasticsearch is available. See ElasticsearchClient.java

If the index pattern is not created correctly, it can lead to errors like this:
stagemonitor/stagemonitor-kibana#4