SOLR-16337: implement Zk metrics

[noblepaul]

copied from #972
implement metrics for ZK ops

How to use?

• start your servers
• hit the end point http://localhost:8983/solr/admin/metrics?key=solr.node:CONTAINER.zkClient

sample output

{
  "metrics":{
    "solr.node:CONTAINER.zkClient":{
      "watchesFired":29,
      "reads":452,
      "writes":171,
      "bytesRead":1260447,
      "bytesWritten":483353,
      "multiOps":26,
      "cumulativeMultiOps":34,
      "childFetches":139,
      "cumulativeChildrenFetched":388,
      "existsChecks":427,
      "deletes":3}}}

[sonatype-lift]

NULL_DEREFERENCE: object returned by getZkClient(zkController) could be null and is dereferenced at line 160.

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