Add splunk logging

[atoulme]

Co-Authored-By: Siegfried Puchbauer siegfried.puchbauer@gmail.com
Signed-off-by: Antoine Toulme antoine@lunar-ocean.com
This adds splunk logging to Besu.

Splunk logging is added as a runtime jar that is pulled from the Spring releases Maven repository.
This PR also removes the Maven repository for the ASF snapshots repository, as no dependencies use this repository at this time.

This change allows users to define Splunk as logger using system properties or environment variables.
This also allows users to change the log level using environment variables if they so choose.

To configure Splunk, you may set the following environment variables:

Name	Description	Required
LOGGER	Must be set to Splunk to activate Splunk logging	Yes
HOST	Current host. If in a Docker environment, the default value is the docker container ID. Otherwise the default value is localhost.	No
SPLUNK_URL	URL to the Splunk instance, for example https://localhost:8088	Yes
SPLUNK_TOKEN	Authentication token, usually of the form 11111111-1111-1111-1111-111111111111	Yes
SPLUNK_INDEX	Index in which logs will be stored. Defaults to besu	No
SPLUNK_SOURCE	Source of the logs. This may be overridden by the Splunk HTTP Event Collector. Defaults to besu	No
SPLUNK_SOURCETYPE	Sourcetype of the logs. This may be overridden by the Splunk HTTP Event Collector. Defaults to besu	No
SPLUNK_BATCH_SIZE_BYTES	Size of a batch in bytes. Defaults to 65536	No
SPLUNK_BATCH_SIZE_COUNT	Size of a batch in number of events. Defaults to 1000	No
SPLUNK_BATCH_INTERVAL	Interval at which to send batches. Defaults to 500	No
SPLUNK_SKIPTLSVERIFY	Whether to check the Splunk instance certificate when sending data. Defaults to false	No

[shemnon]

There is code in the splunk classes that references Logback. Even thought it is a Maven "provided" library because the symbols will show up in the splunk jar and we wind up shipping the splunk we need to be mindful of their (logback's) license. Good news and bad news. EPL 1.0 or LGPL 2.1. Having any sort of an LGPL dependency requires TSC approval. But, the good news. It is an or dependency, at the licensee's choosing. This needs to be enumerated in check-licenses.gradle that we are opting for the EPL1 license. We are already shipping EPL1 libraries (vertx) so that license will add no new restrictions. We already do this decleration for RocksDB as it is also a GPL or Apache situation. So this needs to be made explicit in check-licenses.gradle.

Once this addition is made I can approve it.

[atoulme]

Interesting and noted. Changes have been applied.

[NicolasMassart]

Hi @atoulme could you contact me to help test and write documentation for this feature please?