Fix incorrectly categorized Ecto spans

[mrz]

I use Spandex to send traces to Datadog in two different Elixir projects, using spandex_phoenix and spandex_ecto in conjunction with spandex_datadog. I've noticed that there are some database spans that are incorrectly categorized as Web spans in the spans view, and I've traced the issue back to how spandex_ecto creates the spans. You can see the problem in these screenshots:

As you can see, the first screenshot is showing the Web spans for a trace in Datadog with some Ecto operations. Those are the queue, run_query and decode spans. Switching to the DB spans shows the same operation, but this time it's the query span.

This PR aims to correct this small inconvenience.

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