Adds a mechanism to retain log event metadata such as the severity

[Brice Dutheil]

Drain algorithm is a log mining algorithm, it's idea is to find patterns and group similar log event's message.

The good practice is to make the miner to process only the message part, ie strip elements like the date, the severity, the thread, the name.

Yet it might be interesting to keep some of these information. For example the severity or the log name, are unlikely to have a high cardinality, and maybe good candidate as log cluster metadata.

2021-03-29 12:55:24.172 [] DEBUG --- [  restartedMain] o.s.b.w.s.ServletContextInitializerBeans : Mapping filters: filterRegistrationBean urls=[/*] order=-2147483647, requestContextFilter urls=[/*] order=-1, contextServletRequestFilter urls=[/*] order=-2147483648, characterEncodingFilter urls=[/*] order=-2147483648, edgeRequestContextFilter urls=[/*] order=-2147483646, hideEdgeTechnicalEndpointsFilter urls=[/*] order=-2147483646, enableDebugLogsFilter urls=[/*] order=-2147483645, newrelicTransactionsFilter urls=[/*] order=-2147483645, accountingFilter urls=[/*] order=-2147483644, formContentFilter urls=[/*] order=-9900, disabledForwardedHeaderFilter urls=[/*] order=2147483647
2021-03-29 12:55:24.173 [] DEBUG --- [  restartedMain] o.s.b.w.s.ServletContextInitializerBeans : Mapping servlets: metricsService urls=[/metrics], dispatcherServlet urls=[/rest/*, /doc/*, /actuator/*, /error/*, /favicon.ico], com.blablacar.common.java.web.JerseyConfig urls=[/*]
2021-03-29 12:55:24.554 [] INFO  --- [  restartedMain] o.s.b.a.e.web.EndpointLinksResolver      : Exposing 3 endpoint(s) beneath base path '/actuator'
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2021-03-29 12:55:24.972 [] INFO  --- [  restartedMain] o.s.b.d.a.OptionalLiveReloadServer       : LiveReload server is running on port 35729