chore: fix MSRV breakage

[hawkw]

This branch contains two commits that fix MSRV issues.

• attributes: remove use of non-MSRV-compliant Option::flatten

  PR #[instrument] - support use of the 'self' variable in async-trait contexts #875 added code to tracing-attributes that uses
  Option::flatten, which was only added to the standard library in
  Rust 1.40. This broke our MSRV, but we missed it because the MSRV CI
  checks weren't working correctly (fixed in chore: correct the MSRV check #934).

  This commit removes the use of Option::flatten, and replaces it with
  a manual implementation. It's a little less pretty, but it builds on
  our MSRV (1.39.0).

  Once this merges, we'll publish a new tracing-attributes version,
  and yank 0.1.10.

• 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 chore: correct the MSRV check #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.