Adds NewGaugeFunc and NewCounterFunc (inspired by expvar.Func) which
change the current value to be reported by a function. This allows
some client metric values to be computed on-demand during uploading (at
most every 15 seconds), instead of being continuously updated.
clientmetric uploading had a bunch of micro-optimizations for memory
access (#3331) which are not possible with this approach. However, any
performance hit from function-based metrics is contained to those metrics
only, and we expect to have very few.
Also adds a DisableDeltas() option for client metrics, so that absolute
values are always reported. This makes server-side processing of some
metrics easier to reason about.
Updates tailscale/corp#9230
Signed-off-by: Mihai Parparita <mihai@tailscale.com>
f49b9f75b8