Actix Web OpenTelemetry

OpenTelemetry integration for Actix Web.

Execute client and server example

```console

Run jaeger in background

$ docker run -d -p6831:6831/udp -p6832:6832/udp -p16686:16686 jaegertracing/all-in-one:latest

Run server example with tracing middleware

$ cargo run --example server

(In other tab) Run client example with request tracing

$ cargo run --example client

View spans (see the image below)

$ firefox http://localhost:16686/ ```