[bootstrap] Clarify distinction between service extraction and launching

[smklein]

Basically, had some typos in a previous revision. Updating logs / comments for accuracy.

[smklein]

FYI @bnaecker seeing some test flake related to Oximeter (and, AFAICT, not this patch) on the actions here:

---- test_disks stdout ----
log file: "/tmp/test_disks-05e167aa628d6026-test_disks.18535.0.log"
note: configured to log to "/tmp/test_disks-05e167aa628d6026-test_disks.18535.0.log"
thread 'test_disks' panicked at 'called `Result::unwrap()` on an `Err` value: Failed to discover port from ClickHouse log file', omicron-nexus/tests/common/mod.rs:80:72
error: test failed, to rerun pass '-p omicron-nexus --test test_disks'
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
WARN: dropped CockroachInstance without cleaning it up first (there may still be a child process running and a temporary directory leaked)

Seems related to #130

[bnaecker]

@smklein Thanks for bubbling that up. I've not seen that, either locally or in testing, so I'll dig into it.

[bnaecker]

I think I know what's going on here. We use BufReader to read lines from the ClickHouse log file, searching for one with the port number in it. There's nothing ensuring that the target line is in the output when it's opened and read by this code. It seems likely the reader is hitting EOF before the line is found. I'll try to verify this, and put some checks in place. Thanks @smklein!

[iliana]

Yep, enabling a service sure does happen in BootstrapAgent::enable_service and not here!