feat: SnmpHealthProfile, the OIDs a driver declares; deprecate HealthMetricsMixin (4.12.0)

Drivers no longer collect over SNMP (docs/STRUCTURE.md). A driver whose
device publishes its health values under OIDs of its own declares them
as SNMP_HEALTH_PROFILE: per value either scalars asked in order or a
table folded by first, average or max, optionally with a pattern for
text answers; memory as total with used or free, or a percentage, in
KiB or bytes; and how to read the interfaces. The application polls
and computes.

HealthMetricsMixin.get_health_metrics now warns that it is deprecated;
it goes in 5.0.

Closes #32

README.md

@@ -91,6 +91,23 @@ is a thin bundle over them — `PackageManagementMixin`, `HealthMetricsMixin`,
 `HostRebootMixin` (`reboot_host`) is mixed into `DeviceTypeDriver` itself, since any
 device may be restartable; like the others it only declares.
 
+**SNMP is the application's** (`docs/STRUCTURE.md`). `HealthMetricsMixin` is deprecated
+since 4.12 and goes in 5.0. A driver does not collect over SNMP; a driver whose device
+publishes its health values under OIDs of its own declares them as data:
+
+```python
+class MySwitchDriver(SwitchDriver):
+    SNMP_HEALTH_PROFILE: ClassVar[SnmpHealthProfile] = {
+        "uptime": {"oids": ["1.3.6.1.2.1.1.3.0"]},
+        "cpu": {"oids": ["<vendor cpu oid>", "<older firmware's>"]},
+        "memory": {"total": {"oids": ["<oid>"]}, "free": {"oids": ["<oid>"]}, "unit": "KiB"},
+    }
+```
+
+A value is `oids` (the first that answers wins) or a `table` folded by `first`, `average`
+or `max`, with an optional `pattern` for an agent that answers with text. What the profile
+does not name, the application reads from the standard UCD-MIB and IF-MIB.
+
 `KernelFactsMixin` (`get_kernel_facts`) is the exception that is mixed in by a driver
 rather than by a role base: what a Linux kernel has built and loaded is read the same way
 everywhere, so the command and its parse are concrete here and a driver supplies only