OpenTelemetry integration for Actix Web.

actix-web uses tokio as the underlying executor, so exporters should be configured to be non-blocking:

[dependencies]
# if exporting to jaeger, use the `tokio` feature.
opentelemetry-jaeger = { version = "..", features = ["rt-tokio-current-thread"] }
# if exporting to zipkin, use the `tokio` based `reqwest-client` feature.
opentelemetry-zipkin = { version = "..", features = ["reqwest-client"], default-features = false }
# ... ensure the same same for any other exporters

# 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 --features awc
# View spans (see the image below)
$ firefox https://localhost:16686/

• awc -- enable support for tracing the awc http client.
• metrics -- enable support for opentelemetry metrics (only traces are enabled by default)
• metrics-prometheus -- enable support for prometheus metrics (requires metrics feature)
• sync-middleware -- enable tracing on actix-web middlewares that do synchronous work before returning a future. Adds a small amount of overhead to every request.