Add support for publishing kernel statistics to oximeter (#4355)

- Add a `kstat` module in `oximeter_instruments`. This includes a trait
for describing how to map from one or more kstats into oximeter targets
and metrics. It can be used to convert name-value kernel statistics in a
pretty straightforward way into oximeter samples.
- Add a `KstatSampler`, which is used to register kstat targets, and
will periodically poll them to generate samples. It is an
`oximeter::Producer`, so that it can be easily hooked up to produce data
for an `oximeter` collector.
- Add targets for tracking physical, virtual, and guest datalinks.
- Add metrics for bytes in/out, packets in/out, and errors in/out for
the above.
- Use the `KstatStampler` in a new `MetricsManager` type in the sled
agent, and track the physical (underlay) data links on a system. Does
not yet track any virtual or guest links. The manager can be used to
also collect other statistics, such as HTTP request latencies similar to
nexus, or any kstats through the sampler.

Cargo.toml

@@ -224,6 +224,7 @@ ipcc-key-value = { path = "ipcc-key-value" }
 ipnetwork = { version = "0.20", features = ["schemars"] }
 itertools = "0.11.0"
 key-manager = { path = "key-manager" }
+kstat-rs = "0.2.3"
 lazy_static = "1.4.0"
 libc = "0.2.150"
 linear-map = "1.2.0"