Correctly attribute request's method as HEAD or GET in OT traces when tracing HTTP requests

[alco]

Background: we've recently added support for HEAD requests (#1645) which is implemented by converting any incoming HEAD request into a GET one. That means that Electric builds a response body for such requests even though it's discarded later.

While it might not be a problem performance-wise, and handling HEAD requests this way makes sure the response headers are correct since there's a single execution path for both types of requests, we should still use the original request method in telemetry data that Electric exports (via Open Telemetry and otherwise), so that we can see on performance charts if HEAD requests aren't actually as cheap as we hope they are.