Add documentation about log level (#2108)

Add a few lines about log level configuration.

Author: erthalion

docs/references.md

@@ -96,6 +96,11 @@ bucket (unless they match a defined CIDR-block). Runtime configuration is the
 preferred method to control external IPs, and it overrides this variable.
 Default is disabled.
 
+* `ROX_COLLECTOR_LOG_LEVEL`: Specifies which log level to use, if no logLevel
+is set in the Collector configuration. This is a convenience option: modifying
+Collector configuration might be cumbersome, and setting one environment
+variable is easier.
+
 NOTE: Using environment variables is a preferred way of configuring Collector,
 so if you're adding a new configuration knob, keep this in mind.
 

docs/troubleshooting.md

@@ -55,6 +55,35 @@ following the instructions [here](https://docs.openshift.com/acs/configuration/g
 Within the downloaded bundle, the logs for all Collector pods will be available
 for inspection.
 
+### Log level
+
+Collector features several log levels:
+
+* TRACE: The most verbose one, contains e.g. relocations for the eBPF program
+  and processed events. Not recommended turning on in production due to large
+  performance overhead.
+* DEBUG: Log information essential for debugging, reasonable to use without
+  taking large performance overhead.
+* INFO: The default log level.
+* WARNING: Contains information about unexpected situations.
+* ERROR: Reports processing failures, without stopping Collector.
+* FATAL: Unrecoverable errors, duplicated to the termination log.
+
+There are a few ways to configure log level in Collector, all are
+case-insensitive:
+
+* Via environment variable `ROX_COLLECTOR_LOG_LEVEL`.
+
+* In the Collector configuration via the `logLevel` key. This option takes
+  precedence over the environment variable.
+
+* Via API call `/loglevel`. This will enforce the log level value, regardless
+  of existing configuration.
+
+```
+$ curl -X POST -d "trace" collector:8080/loglevel
+```
+
 ### Pod Status
 
 Another way of getting a quick look at the reason for a crashing Collector is to