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.
- Enabled process exporter for storage services
- Changed zenith_proxy prefix to just proxy
- Removed old `monitoring` directory
- Removed common prefix for metrics, now our common metrics have `libmetrics_` prefix, for example `libmetrics_serve_metrics_count`
- Added `test_metrics_normal_work`