Add span opts in router dispatch

[davidjulien]

Hello,

When you associate a different service name for phoenix, the phx.router_dispatch has the default service name and not the custom one.

The first commit allows to fix that by adding the span opts when the span for phx.router_dispatch is started.

In case of an exception, phx.router_dispatch uses again the default service name:

The second commit allows to fix that by adding the span opts when the span is finished:

We use this fix in prod and it works well.

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[GregMefford]

Hey, sorry it took so long to get this merged, but thanks for taking the time to open this PR! 🎈 🚀