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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.