Reduce debug logging when sanitizing documents with multifields

[jsoriano]

Before this change, a debug message was printed for every multifield. Multifields can be pretty common,
specially after the introduction of ecs@mappings, what in some cases floods the logs with lines like these ones:

...
not able to add key threat.indicator.as.organization.name.text, is this a multifield?: expected map but type is string
not able to add key agent.name.text, is this a multifield?: expected map but type is string
not able to add key agent.name.text, is this a multifield?: expected map but type is string
not able to add key agent.name.text, is this a multifield?: expected map but type is string
...

With this change a single line is printed every time the method is called:

Some keys were not included in sanitized docs because the are multifields: agent.name.text, threat.indicator.as.organization.name.text, threat.indicator.url.full.text, threat.indicator.url.original.text

Other failed cases are printed at the warning level.

[elasticmachine]

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• Commit: 37bce43

cc @jsoriano