[Metrics] Align Metric and Span duration for timing API

[philipphofmann]

Description

The metric and span for a call to the metrics timing method might not have the same duration. Starting and finishing the span and measuring the duration in the aggregator for the callback use two distinct calls to get the current time. Therefore, it's highly likely that the duration of the span and the metric don't match in some cases.

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

Lines 431 to 443 in fde534e

	final @Nullable ISpan span = aggregator.startSpanForMetric("metric.timing", key);
	if (span != null && tags != null) {
	for (final @NotNull Map.Entry<String, String> entry : tags.entrySet()) {
	span.setTag(entry.getKey(), entry.getValue());
	}
	}
	try {
	aggregator
	.getMetricsAggregator()
	.timing(key, callback, durationUnit, enrichedTags, localMetricsAggregator);
	} finally {
	if (span != null) {
	span.finish();

[philipphofmann]

@stefanosiano, when you add the timing API for Dart you could already take this into account ⬆️.