Unify RedactionUtils and metrics hiddenSysProps settings

System properties can contain sensitive data, and they are easily available from the Admin UI (/admin/info/system) and also from the Metrics API (/admin/metrics).

By default the /admin/info/system redacts any sys prop with a key containing password. This can be configured with sysprop -Dsolr.redaction.system.pattern=<regex>

The metrics API by default hides these sysprops from the API output:

    "javax.net.ssl.keyStorePassword",
    "javax.net.ssl.trustStorePassword",
    "basicauth",
    "zkDigestPassword",
    "zkDigestReadonlyPassword"

You can redefine these by adding a section to solr.xml: (https://lucene.apache.org/solr/guide/7_5/metrics-reporting.html#the-metrics-hiddensysprops-element)

<metrics>
 <hiddenSysProps>
   <str>foo</str>
   <str>bar</str>
   <str>baz</str>
 </hiddenSysProps>
</metrics>

Unifying the two

It is not very user firiendly to have two different systems for redacting system properties and two sets of defaults. This goals of this issue are

• Keep only one set of defaults
• Both metrics and system info handler will use the same source
• It should be possible to change and persist the list without a full cluster restart, preferably though some API

Note that the solr.redaction.system.pattern property is not documented in the ref guide, so this Jira should also fix documentation!