This splits Prometheus metric handlers exposed by tsweb into two
modules:
- `varz.Handler` exposes Prometheus metrics generated by our expvar
converter;
- `promvarz.Handler` combines our expvar-converted metrics and native
Prometheus metrics.
By default, tsweb will use the promvarz handler, however users can keep
using only the expvar converter. Specifically, `tailscaled` now uses
`varz.Handler` explicitly, which avoids a dependency on the
(heavyweight) Prometheus client.
Updates https://github.com/tailscale/corp/issues/10205
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
Anton Tolchanov