Add JVM Metrics to Prom Exporter

## What?

The current exporter doesn't contain some of the metrics exposed in the graylog-labs/graylog-plugin-metrics-reporter project. See the metrics in the dump here:
[metrics_reporter_promethus.txt](https://github.com/Graylog2/graylog2-server/files/7063568/metrics_reporter_promethus.txt)

The specific metrics are:

```
# HELP jvm_gc_ConcurrentMarkSweep_count Generated from Dropwizard metric import (metric=jvm.gc.ConcurrentMarkSweep.count, type=com.codahale.metrics.jvm.GarbageCollectorMetricSet$$Lambda$37/786669171)
# TYPE jvm_gc_ConcurrentMarkSweep_count gauge
jvm_gc_ConcurrentMarkSweep_count 8.0
# HELP jvm_gc_ConcurrentMarkSweep_time Generated from Dropwizard metric import (metric=jvm.gc.ConcurrentMarkSweep.time, type=com.codahale.metrics.jvm.GarbageCollectorMetricSet$$Lambda$38/124351398)
# TYPE jvm_gc_ConcurrentMarkSweep_time gauge
jvm_gc_ConcurrentMarkSweep_time 168.0
# HELP jvm_gc_ParNew_count Generated from Dropwizard metric import (metric=jvm.gc.ParNew.count, type=com.codahale.metrics.jvm.GarbageCollectorMetricSet$$Lambda$37/786669171)
# TYPE jvm_gc_ParNew_count gauge
jvm_gc_ParNew_count 114.0
# HELP jvm_gc_ParNew_time Generated from Dropwizard metric import (metric=jvm.gc.ParNew.time, type=com.codahale.metrics.jvm.GarbageCollectorMetricSet$$Lambda$38/124351398)
# TYPE jvm_gc_ParNew_time gauge
jvm_gc_ParNew_time 850.0
```

This relates to #10810