[9.0] fix: do not overwrite registries when publishing otel metrics to beats (backport #15517)

[mergify]

Motivation/summary

update apm-server registry mapping logic to avoid overwriting onkey handlers

discovered while backporting the otel changes. Already added in 8.x

if metrics have similar paths the onkey handler is overwritten. To avoid this process all the metrics path first to build a tree and produce them in order.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

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This is an automatic backport of pull request #15517 done by [Mergify](https://mergify.com).