[Perf][Metrics] Use flurry's concurrent hashmap for 5x throughput#2305

utpilla wants to merge 1 commit intoopen-telemetry:mainopen-telemetry/opentelemetry-rust:mainfrom

utpilla:utpilla/Use-concurrent-hashmap-for-metricsutpilla/opentelemetry-rust:utpilla/Use-concurrent-hashmap-for-metricsCopy head branch name to clipboard