Fix errors and warnings for the .reflect_shared() example

[nightkr]

Doesn't compile with the & (nonsense both type-wise and lifetime-wise), might as well fix the other warnings while at it. Also moved the subscription into a concurrent task since it's unreachable otherwise.

Also not sure about whether we want to add a tracing init too.. on one hand it's kind of tangential to the main thing being demonstrated, on the other hand it's easy to get confused by the empty output if you're not used to how tracing works.

Discovered while looking into https://discord.com/channels/500028886025895936/1302071483585269771.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.3%. Comparing base (a0fcd1e) to head (71de1bd).
Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1625   +/-   ##
=====================================
  Coverage   75.3%   75.3%           
=====================================
  Files         82      82           
  Lines       7344    7344           
=====================================
  Hits        5528    5528           
  Misses      1816    1816           

Files with missing lines	Coverage Δ
kube-runtime/src/utils/watch_ext.rs	22.3% <ø> (ø)

[clux]

Ah, good catch. I guess we expected the subscriber to normally be polled through a second controller, as in the shared stream example these are ultimately polled via a select! arm.