fix(metrics): Re-introduce transactions namespace

[Dav1dde]

#5792 removed transaction metric extraction as well as the transactions metric namespace.

What was kind of missed that there are still a lot of on demand metric extraction rules generated in Sentry which create metrics in the transactions namespace. This leads to Relay extracting the metrics into an unsupported namespace.

To mitigate this we can re-introduce the transaction namespace and work on removing the on-demand rules from Sentry first.

[cursor]

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