• Added ICounter.NewTimer() (adds the value to the counter)
• Eliminated some needless allocations when trying to register a metric that is already registered. #134
• Added IHistogram.Count and IHistogram.Sum to expose aspects of collected data for inspection.
• Added Collector.GetAllLabelValues() to expose the list of child metrics by all their known label values.
• Metric export is now asynchronous internally to be compatible with ASP.NET Core 3.0 default configuration.
• Added CollectorRegistry.CollectAndExportAsTextAsync() to support metric data export via arbitrary custom endpoints.