introduce LaunchTimestamp to identify process restarts

This patch adds a LaunchTimestamp type to the `metrics` crate,
along with a `libmetric_` Prometheus metric.

The initial user is pageserver.
In addition to exposing the Prometheus metric, it also reproduces
the launch timestamp as a header in the API responses.

The motivation for this is that we plan to scrape the pageserver's
/v1/tenant/:tenant_id/timeline/:timeline_id/layer
HTTP endpoint over time. It will soon expose access metrics (#3496)
which reset upon process restart. We will use the pageserver's launch
ID to identify a restart between two scrape points.

However, there are other potential uses. For example, we could use
the Prometheus metric to annotate Grafana plots whenever the launch
timestamp changes.

Cargo.lock

@@ -2019,6 +2019,7 @@ dependencies = [
 name = "metrics"
 version = "0.1.0"
 dependencies = [
+ "chrono",
  "libc",
  "once_cell",
  "prometheus",