fix concurrent map write error caused by reusing a mapstr.M

[tommyers-elastic]

In the existing code, we create a mapstr.M containing attributes to set in the RootFields of metric events, which is reused several times in a loop. In the publisher code, the root fields are modified, e.g. https://github.com/elastic/beats/blob/main/metricbeat/mb/event.go#L79, which causes a panic due to fatal error: concurrent map iteration and map write, as the same mapstr.M object is still being used in later iterations of the loop.

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