Fix compiler warnings with nightly rustc about elided lifetimes having names

[hlinnaka]

The warnings:

warning: elided lifetime has a name
    --> pageserver/src/metrics.rs:1386:29
     |
1382 |     pub(crate) fn start_timer<'c: 'a, 'a>(
     |                                       -- lifetime `'a` declared here
...
1386 |     ) -> Option<impl Drop + '_> {
     |                             ^^ this elided lifetime gets resolved as `'a`
     |
     = note: `#[warn(elided_named_lifetimes)]` on by default

warning: elided lifetime has a name
    --> pageserver/src/metrics.rs:1537:46
     |
1534 |     pub(crate) fn start_recording<'c: 'a, 'a>(
     |                                           -- lifetime `'a` declared here
...
1537 |     ) -> BasebackupQueryTimeOngoingRecording<'_, '_> {
     |                                              ^^ this elided lifetime gets resolved as `'a`

warning: elided lifetime has a name
    --> pageserver/src/metrics.rs:1537:50
     |
1534 |     pub(crate) fn start_recording<'c: 'a, 'a>(
     |                                           -- lifetime `'a` declared here
...
1537 |     ) -> BasebackupQueryTimeOngoingRecording<'_, '_> {
     |                                                  ^^ this elided lifetime gets resolved as `'a`

warning: elided lifetime has a name
    --> pageserver/src/tenant.rs:3630:25
     |
3622 |     async fn prepare_new_timeline<'a>(
     |                                   -- lifetime `'a` declared here
...
3630 |     ) -> anyhow::Result<UninitializedTimeline> {
     |                         ^^^^^^^^^^^^^^^^^^^^^ this elided lifetime gets resolved as `'a`

[github-actions]

5029 tests run: 4865 passed, 0 failed, 164 skipped (full report)

---

Flaky tests (5)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_hot_standby_feedback: debug-x86-64
• test_cli_start_stop_multi: release-x86-64
• test_pg_regress[None]: debug-x86-64
• test_scrubber_physical_gc_ancestors_split: debug-x86-64

Code coverage* (full report)

• functions: 32.1% (7447 of 23213 functions)
• lines: 49.9% (59952 of 120174 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
5bd58bc at 2024-09-23T21:04:38.033Z :recycle:}