examples: disable opentelemetry's default features

The `opentelemetry` crate depends on `prometheus`, which depends on
`protobuf`, a crate which doesn't compile on our MSRV (Rust 1.39). This
was missed due to issues with the MSRV CI checks, which will be fixed
fixed in #934. Therefore, once the MSRV checks work properly, the
`protobuf` dependency will break our builds.

We don't _need_ the `opentelemetry/metrics` feature, which is what
enables the `prometheus` (and thus `protobuf`) dependency.
`tracing-opentelemetry` already has a `default-features = false`
dependency on `opentelemetry`, but the examples don't. Therefore, I've
changed the examples crate to disable `opentelemetry`'s default features
as well.

examples/Cargo.toml

@@ -52,5 +52,5 @@ inferno = "0.10.0"
 tempdir = "0.3.7"
 
 # opentelemetry example
-opentelemetry = "0.8"
+opentelemetry = { version = "0.8", default-features = false, features = ["trace"] }
 opentelemetry-jaeger = "0.7"