opentelemetry: add more comments to example

[bryangarza]

This patch adds a bit more context around why we are creating a smaller
scope for the spans, and also what happens when we call
global::shutdown_tracer_provider() (that comment was copied from
therust-opentelemetry repo).

[hawkw]

This looks good to me. I'm not totally sure why the RustDoc CI build is failing --- it builds fine on my machine.

Also, should we consider adding similar code/comments to the examples in the README and RustDoc for tracing-opentelemetry? Here:

tracing/tracing-opentelemetry/README.md

Lines 67 to 86 in 604f389

	fn main() {
	// Install a new OpenTelemetry trace pipeline
	let (tracer, _uninstall) = stdout::new_pipeline().install();
	// Create a tracing subscriber with the configured tracer
	let telemetry = tracing_opentelemetry::subscriber().with_tracer(tracer);
	// Use the tracing subscriber `Registry`, or any other subscriber
	// that impls `LookupSpan`
	let collector = Registry::default().with(telemetry);
	// Trace executed code
	tracing::collect::with_default(collector, || {
	// Spans will be sent to the configured OpenTelemetry exporter
	let root = span!(tracing::Level::TRACE, "app_start", work_units = 2);
	let _enter = root.enter();
	error!("This event will be logged in the root span.");
	});
	}

and here:

tracing/tracing-opentelemetry/src/lib.rs

Lines 54 to 76 in 604f389

	//! use opentelemetry::sdk::export::trace::stdout;
	//! use tracing::{error, span};
	//! use tracing_subscriber::subscribe::CollectExt;
	//! use tracing_subscriber::Registry;
	//!
	//! // Create a new OpenTelemetry pipeline
	//! let tracer = stdout::new_pipeline().install_simple();
	//!
	//! // Create a tracing layer with the configured tracer
	//! let telemetry = tracing_opentelemetry::subscriber().with_tracer(tracer);
	//!
	//! // Use the tracing subscriber `Registry`, or any other subscriber
	//! // that impls `LookupSpan`
	//! let subscriber = Registry::default().with(telemetry);
	//!
	//! // Trace executed code
	//! tracing::collect::with_default(subscriber, || {
	//! // Spans will be sent to the configured OpenTelemetry exporter
	//! let root = span!(tracing::Level::TRACE, "app_start", work_units = 2);
	//! let _enter = root.enter();
	//!
	//! error!("This event will be logged in the root span.");
	//! });

[hawkw]

looks good to me now that the CI build passes! would be nice to also add to the RustDoc examples, like i mentioned in #2140 (review)