integration: fix missing traces

[hawkw]

Currently, there are a couple of issues that prevent traces emitted by
the proxy's integration tests from being reported correctly. Many of our
tests begin with:

let _ = trace_init();

This doesn't actually do what we want, for two reasons:

• trace_init doesn't actually set a trace subscriber as the default,
  it just constructs one,
• let _ drops the value immediately.

So, this code is constructing a subscriber and then immediately dropping
it, which does nothing.

In practice, we mostly don't notice this, since the test support
servers, clients, and proxy run in separate threads with their own
trace subscribers, which are set up correctly, and very few of the tests
themselves actually emit traces. So even though this does the wrong
thing, everything basically works. However, if the dedicated test
support threads are removed and the test support stuff moved to spawned
tasks in the test functions, all the traces go away.

This PR fixes that by making trace_init actually set a default
subscriber, adding a new trace_subscriber function that just
constructs the subscriber and level handle (which is necessary to run a
test proxy), and changing the tests so they don't drop their guards
until the test ends.

Signed-off-by: Eliza Weisman [email protected]

[kleimkuhler]

There are a lot of double semicolons in discovery.rs and one in profile.rs. Locally it runs fine without them, so unless I'm missing something those can be removed.