use statsd for indexer monitoring

[vladak]

This change introduces remote monitoring of indexer via statsd registry in Micrometer. I wanted to use statsd for indexer because it does not require scraping and therefore the need for a web server. This would either conflict with already running web server on the same machine (assuming the indexer is usually run on the same machine as the web application) or would involve poking holes into firewall to allow the scraping to pass through to distinct TCP port.

Example read-only configuration:

<?xml version="1.0" encoding="UTF-8"?>
<java version="11.0.4" class="java.beans.XMLDecoder">
 <object class="org.opengrok.indexer.configuration.Configuration" id="Configuration0">

  <void property="statsdConfig">
     <void property="port">
       <int>8125</int>
     </void>
     <void property="host">
       <string>localhost</string>
     </void>
     <void property="flavor">
       <string>etsy</string>
     </void>
  </void>

 </object>
</java>

This was used to generate the graphs in #3248 (comment) (I used statsd-exporter for Prometheus and Grafana)

The metrics are not tagged unless the indexer is run per project.

During testing I noticed that some of the metrics recorded in the final stages of indexing are not sent out even with buffering set to false and big delay at the end of the Indexer#main. This is not such a big deal given that statsd traffic is usually not reliable anyway (assuming UDP by default). I will definitely investigate.

Once this is merged in I will update https://github.com/oracle/opengrok/wiki/Monitoring with example rig configuration.

[coveralls]

Pull Request Test Coverage Report for Build 5493

• 75 of 97 (77.32%) changed or added relevant lines in 12 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.005%) to 75.7%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/Configuration.java	5	6	83.33%
opengrok-web/src/main/java/org/opengrok/web/servlet/MetricsServlet.java	0	1	0.0%
opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/RuntimeEnvironment.java	6	8	75.0%
opengrok-web/src/main/java/org/opengrok/web/StatisticsFilter.java	0	4	0.0%
opengrok-indexer/src/main/java/org/opengrok/indexer/Metrics.java	21	35	60.0%

Files with Coverage Reduction	New Missed Lines	%
opengrok-indexer/src/main/java/org/opengrok/indexer/history/BazaarRepository.java	2	69.0%

Totals
Change from base Build 5478:	-0.005%
Covered Lines:	41401
Relevant Lines:	54691

---

💛 - Coveralls

[vladak]

Thanks for the review !