Add PuppetDB Jetty thread metrics

[jarretlavallee]

Prior to this PR, there were no metrics around the jetty thread
pool. This PR adds additional metrics to collect the jetty
queuedthreadpool metrics. This allows for looking at the jetty thread
usage in PuppetDB.

Example:

                "jetty-queuedthreadpool": {
                    "org.eclipse.jetty.util.thread:id=0,type=queuedthreadpool": {
                        "availableReservedThreads": 2,
                        "busyThreads": 7,
                        "daemon": false,
                        "detailedDump": false,
                        "idleThreads": 1,
                        "idleTimeout": 60000,
                        "leasedThreads": 6,
                        "lowOnThreads": false,
                        "lowThreadsThreshold": 1,
                        "maxAvailableThreads": 194,
                        "maxLeasedThreads": 10,
                        "maxReservedThreads": 4,
                        "maxThreads": 200,
                        "minThreads": 8,
                        "name": "qtp1512214559",
                        "queueSize": 0,
                        "readyThreads": 3,
                        "reservedThreads": -1,
                        "state": "STARTED",
                        "stopTimeout": 30000,
                        "threads": 10,
                        "threadsPriority": 5,
                        "utilizationRate": 0.005154639175257732,
                        "utilizedThreads": 1
                    }
                },

[puppet-community-rangefinder]

puppet_metrics_collector::service::puppetdb is a class

that may have no external impact to Forge modules.

This module is declared in 0 of 576 indexed public Puppetfiles.

---

These results were generated with Rangefinder, a tool that helps predict the downstream impact of breaking changes to elements used in Puppet modules. You can run this on the command line to get a full report.

Exact matches are those that we can positively identify via namespace and the declaring modules' metadata. Non-namespaced items, such as Puppet 3.x functions, will always be reported as near matches only.

[MartyEwings]

tested in my 2021.1 box new metric available

 },
        "jetty-queuedthreadpool": {
          "org.eclipse.jetty.util.thread:id=0,type=queuedthreadpool": {
            "threadsPriority": 5,
            "lowOnThreads": false,
            "queueSize": 0,
            "availableReservedThreads": 3,
            "stopTimeout": 30000,
            "utilizedThreads": 1,
            "utilizationRate": 0.005154639175257732,
            "minThreads": 8,
            "detailedDump": false,
            "maxReservedThreads": 4,
            "threads": 40,
            "reservedThreads": -1,
            "readyThreads": 33,
            "leasedThreads": 6,
            "daemon": false,
            "maxAvailableThreads": 194,
            "busyThreads": 7,
            "lowThreadsThreshold": 1,
            "idleTimeout": 60000,
            "name": "qtp2004632949",
            "idleThreads": 30,
            "state": "STARTED",
            "maxThreads": 200,
            "maxLeasedThreads": 10
          }

Unable to see anything broken