Adopt OpenTelemetry's semantic convention (#15)

* Add http.route.

* Align all fields with OpenTelemetry's semantic conventions.

* Add span kind.

* Emit event for errors.
Add OTEL status code.

* Create otel.status_code field as empty.

* Fix errors.

* Add (feature-gated) support for OpenTelemetry span propagation.

* Capture the trace id as an attribute on the span.

* Change message.

* Log the newly-generated trace id if there is no parent context.

* Define a root_span macro as a stepping stone to allow crate users to add their own fields to the root span.

* Add comments.

* mut is no longer necessary.

* Allow users to customise generation of the root span. Split recording fields on span end from emission of log record. Make log record on error optional via feature flag.

* Provide constructor + default implementation.

* Explode into multiple modules.
Fix various paths/private imports in root_span.

* Rename module to root_span_macro.

* Add a new extractor to retrieve the root span.

* Document crate.

* Docs!

* Add section on OTEL.

* Mention actix-web-opentelemetry.

* Add OpenTelemetry example.

* Improve readme.

* Add custom root span example.

Co-authored-by: LukeMathWalker <contact@palmieri.com>

examples/custom-root-span/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "custom-root-span"
+version = "0.1.0"
+authors = ["LukeMathWalker <contact@lpalmieri.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+actix-web = "4.0.0-beta.6"
+opentelemetry = { version = "0.13", features = ["rt-tokio-current-thread"] }
+opentelemetry-jaeger = { version = "0.12", features = ["tokio"] }
+tracing-opentelemetry = { version = "0.12" }
+tracing = "0.1.19"
+tracing-subscriber = { version = "0.2.12", features = ["registry", "env-filter"] }
+tracing-bunyan-formatter = "0.1.6"
+tracing-actix-web = { path = "../.." }