Add metrics for transport layer in instrumentation net/http

[kw510]

Metrics are provided in the handler of net/http, however they are not provided in the transport layer. This would be super useful to monitor outgoing requests.

https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/instrumentation/net/http/otelhttp/transport.go

[rangzen]

I invite you to follow #3765 to avoid duplicating the actual problem when resolving this issue.

[Sovietaced]

@MrAlias Is this an issue you would be open to having a community contribution for? I find it strange that client side metrics are not present for HTTP/gRPC. Server side metrics are only useful if the client can reach the server.

[jlordiales]

@Sovietaced there's already an open PR for this: #3769

[Sovietaced]

Thanks @jlordiales

I will focus my efforts there.