Collect JVM memory metrics

[jicelhay]

Adds JVM memory monitoring by registering new gauge metrics into the existing MetricRegistryImpl object. Metrics in MetricRegistryImpl are already being pushed to Cloud monitoring every 60 secs by a MetricReporter.

Example of metrics in crash: https://console.cloud.google.com/monitoring/metrics-explorer;duration=PT1H?referrer=search&project=domain-registry-crash&pageState=%7B%22xyChart%22:%7B%22constantLines%22:%5B%5D,%22dataSets%22:%5B%7B%22plotType%22:%22LINE%22,%22pointConnectionMethod%22:%22GAP_DETECTION%22,%22targetAxis%22:%22Y1%22,%22timeSeriesFilter%22:%7B%22aggregations%22:%5B%7B%22alignmentPeriod%22:%2260s%22,%22crossSeriesReducer%22:%22REDUCE_SUM%22,%22groupByFields%22:%5B%22metric.label.%5C%22type%5C%22%22,%22resource.label.%5C%22pod_id%5C%22%22%5D,%22perSeriesAligner%22:%22ALIGN_MEAN%22%7D%5D,%22apiSource%22:%22DEFAULT_CLOUD%22,%22crossSeriesReducer%22:%22REDUCE_SUM%22,%22filter%22:%22metric.type%3D%5C%22custom.googleapis.com%2Fjvm%2Fmemory%2Fmax%5C%22%20resource.type%3D%5C%22gke_container%5C%22%22,%22groupByFields%22:%5B%22metric.label.%5C%22type%5C%22%22,%22resource.label.%5C%22pod_id%5C%22%22%5D,%22minAlignmentPeriod%22:%2260s%22,%22perSeriesAligner%22:%22ALIGN_MEAN%22%7D%7D%5D,%22options%22:%7B%22mode%22:%22COLOR%22%7D,%22y1Axis%22:%7B%22label%22:%22%22,%22scale%22:%22LINEAR%22%7D%7D%7D

b/468031702

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This change is