walredo: stderr cleanup & make explicitly cancel safe

[problame]

Problem

I need walredo to be cancellation-safe for #6000 (comment)

Solution

We are only async fn because of wait_for(stderr_logger_task_done).await, added in #5560 .

The stderr_logger_cancel and stderr_logger_task_done were there out of precaution that the stderr logger task might for some reason not stop when the walredo process terminates.
That hasn't been a problem in practice.
So, simplify things:

• remove stderr_logger_cancel and the wait_for(...stderr_logger_task_done...)
• use tokio::process::ChildStderr in the stderr logger task
• add metrics to track number of running stderr logger tasks so in case I'm wrong here, we can use these metrics to identify the issue (not planning to put them into a dashboard or anything)

[github-actions]

2436 tests run: 2340 passed, 0 failed, 96 skipped (full report)

---

Flaky tests (4)

Postgres 16

• test_crafted_wal_end[last_wal_record_xlog_switch_ends_on_page_boundary]: release

Postgres 15

• test_competing_branchings_from_loading_race_to_ok_or_err: release
• test_branching_with_pgbench[cascade-1-10]: debug

Postgres 14

• test_ondemand_download_timetravel[real_s3]: debug

Code coverage (full report)

• functions: 54.6% (9297 of 17027 functions)
• lines: 82.1% (54016 of 65796 lines)

---

_{The comment gets automatically updated with the latest test results
b1c0dd7 at 2023-12-04T16:00:49.188Z :recycle:}