Improve error handling when connection to InfluxDB cannot be established.

[mariusoe]

In the current version, the agent log (also the application log) is getting a large amount of exception message due to a failing InfluxDB connection.

One possible solution would be to log the stack trace only in the debug level.

Example, in case the InfluxDB is not availabe, the agent will log the following message every 15 seconds (the default metric export frequency).

2020-03-04 15:04:16,947 ERROR 314347 --- [inspectIT] [pectit-thread-1] r.i.opencensus.influx.InfluxExporter     : Could not connect to influx
java.net.ConnectException: Connection refused: connect
	at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method)
	at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:85)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
	at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
	at java.net.Socket.connect(Socket.java:589)
	at okhttp3.internal.platform.Platform.connectSocket(Platform.java:130)
	at okhttp3.internal.connection.RealConnection.connectSocket(RealConnection.java:247)
	... 42 common frames omitted
Wrapped by: java.net.ConnectException: Failed to connect to localhost/0:0:0:0:0:0:0:1:8086
	at okhttp3.internal.connection.RealConnection.connectSocket(RealConnection.java:249)
	at okhttp3.internal.connection.RealConnection.connect(RealConnection.java:167)
	at okhttp3.internal.connection.StreamAllocation.findConnection(StreamAllocation.java:257)
	at okhttp3.internal.connection.StreamAllocation.findHealthyConnection(StreamAllocation.java:135)
	at okhttp3.internal.connection.StreamAllocation.newStream(StreamAllocation.java:114)
	at okhttp3.internal.connection.ConnectInterceptor.intercept(ConnectInterceptor.java:42)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
	at okhttp3.internal.cache.CacheInterceptor.intercept(CacheInterceptor.java:94)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
	at okhttp3.internal.http.BridgeInterceptor.intercept(BridgeInterceptor.java:93)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
	at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.java:125)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
	at org.influxdb.impl.BasicAuthInterceptor.intercept(BasicAuthInterceptor.java:22)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
	at org.influxdb.impl.GzipRequestInterceptor.intercept(GzipRequestInterceptor.java:42)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
	at okhttp3.logging.HttpLoggingInterceptor.intercept(HttpLoggingInterceptor.java:152)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
	at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:264)
	at okhttp3.RealCall.execute(RealCall.java:93)
	at retrofit2.OkHttpCall.execute(OkHttpCall.java:186)
	at org.influxdb.impl.InfluxDBImpl.execute(InfluxDBImpl.java:800)
	... 14 common frames omitted
Wrapped by: org.influxdb.InfluxDBIOException: java.net.ConnectException: Failed to connect to localhost/0:0:0:0:0:0:0:1:8086
	at org.influxdb.impl.InfluxDBImpl.execute(InfluxDBImpl.java:812)
	at org.influxdb.impl.InfluxDBImpl.executeQuery(InfluxDBImpl.java:795)
	at org.influxdb.impl.InfluxDBImpl.query(InfluxDBImpl.java:552)
	at rocks.inspectit.opencensus.influx.InfluxExporter.connectAndCreateDatabase(InfluxExporter.java:216)
	at rocks.inspectit.opencensus.influx.InfluxExporter.export(InfluxExporter.java:79)
	at rocks.inspectit.opencensus.influx.InfluxExporter.export(InfluxExporter.java:72)
	at io.grpc.Context$1.run(Context.java:595)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.runAndReset$$$capture(FutureTask.java:308)
	at java.util.concurrent.FutureTask.runAndReset(FutureTask.java)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)