Clarify the slow flush warning

[codebien]

We should clarify the warning message when a single flush iteration is slow by adding two values with the time required for aggregating and flushing. In this way, it is easier for users to identify where is the potential issue.

xk6-output-prometheus-remote/pkg/remotewrite/remotewrite.go

Lines 205 to 207 in c80a73f

	o.logger.WithField("nts", nts).
	Warnf("%s but it took %s while flush period is %s. Some samples may be dropped.",
	okmsg, d.String(), o.config.PushInterval.String())