[oximeter] Add some units from physical reality

In service of future changes to record data from power, temperature, and
fan speed sensors in Oximeter, this branch adds some physical quantities
to the `Units` enum: `Volts`, `Amps`, `DegreesCelcius`, and `Rpm`. I've
added all of these as whole numbers of the measured quantities, with the
expectation that they will probably be recorded as floating-point. We
could consider instead using a smaller unit like `MilliAmps`, and
recording them as integers, but that introduces a bunch of dimensional
analysis that I'm not sure if we want to be doing.

oximeter/impl/src/schema/codegen.rs

@@ -522,6 +522,12 @@ impl quote::ToTokens for Units {
             Units::Nanoseconds => {
                 quote! { ::oximeter::schema::Units::Nanoseconds }
             }
+            Units::Amps => quote! { ::oximeter::schema::Units::Amps },
+            Units::Volts => quote! { ::oximeter::schema::Units::Volts },
+            Units::DegreesCelcius => {
+                quote! { ::oximeter::schema::Units::DegreesCelcius }
+            }
+            Units::Rpm => quote! { ::oximeter::schema::Units::Rpm  },
         };
         toks.to_tokens(tokens);
     }

oximeter/impl/src/schema/mod.rs

@@ -179,7 +179,6 @@ pub struct TimeseriesDescription {
 /// Measurement units for timeseries samples.
 #[derive(Clone, Copy, Debug, Deserialize, Serialize, JsonSchema, PartialEq)]
 #[serde(rename_all = "snake_case")]
-// TODO-completeness: Include more units, such as power / temperature.
 // TODO-completeness: Decide whether and how to handle dimensional analysis
 // during queries, if needed.
 pub enum Units {
@@ -189,6 +188,11 @@ pub enum Units {
     Bytes,
     Seconds,
     Nanoseconds,
+    Volts,
+    Amps,
+    DegreesCelcius,
+    /// Rotations per minute.
+    Rpm,
 }
 
 /// The schema for a timeseries.