[Metrics] Timing API race condition

[philipphofmann]

Description

If the hub's current span changes between these two calls, then LocalMetricsAggregator doesn't match the metric's started span.

sentry-java/sentry/src/main/java/io/sentry/metrics/MetricsApi.java

Lines 428 to 431 in fde534e

	final @Nullable LocalMetricsAggregator localMetricsAggregator =
	aggregator.getLocalMetricsAggregator();
	final @Nullable ISpan span = aggregator.startSpanForMetric("metric.timing", key);