feat: Add description getter to metrics::Instrument in SDK

This change is in conflict with
<#2985>:

> Instrument currently allows users to select an instrument based on
> description, which is not allowed in the spec. Only name, kind, unit, scope
> should be exposed.

The spec at
<https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#instrument-selection-criteria>
says:

> The SDK MAY accept additional criteria.

So it seems like the description MAY be exposed.

This commit also updates docs based on the field definitions, which seem more
descriptive, and cleans up some others.

Author: ongardie-atomix

opentelemetry-sdk/src/metrics/instrument.rs

@@ -32,12 +32,10 @@ pub enum InstrumentKind {
     /// A group of instruments that record increasing and decreasing values in an
     /// asynchronous callback.
     ObservableUpDownCounter,
-
-    /// a group of instruments that record current value synchronously with
+    /// A group of instruments that record current value synchronously with
     /// the code path they are measuring.
     Gauge,
-    ///
-    /// a group of instruments that record current values in an asynchronous callback.
+    /// A group of instruments that record current values in an asynchronous callback.
     ObservableGauge,
 }
 
@@ -97,33 +95,38 @@ impl InstrumentKind {
 pub struct Instrument {
     /// The human-readable identifier of the instrument.
     pub(crate) name: Cow<'static, str>,
-    /// describes the purpose of the instrument.
+    /// Describes the purpose of the instrument.
     pub(crate) description: Cow<'static, str>,
     /// The functional group of the instrument.
     pub(crate) kind: InstrumentKind,
-    /// Unit is the unit of measurement recorded by the instrument.
+    /// The unit of measurement recorded by the instrument.
     pub(crate) unit: Cow<'static, str>,
-    /// The instrumentation that created the instrument.
+    /// Information about the library that created the instrument.
     pub(crate) scope: InstrumentationScope,
 }
 
 impl Instrument {
-    /// Instrument name.
+    /// The human-readable identifier of the instrument.
     pub fn name(&self) -> &str {
         self.name.as_ref()
     }
 
-    /// Instrument kind.
+    /// Describes the purpose of the instrument.
+    pub fn descripton(&self) -> &str {
+        self.description.as_ref()
+    }
+
+    /// The functional group of the instrument.
     pub fn kind(&self) -> InstrumentKind {
         self.kind
     }
 
-    /// Instrument unit.
+    /// The unit of measurement recorded by the instrument.
     pub fn unit(&self) -> &str {
         self.unit.as_ref()
     }
 
-    /// Instrument scope.
+    /// Information about the library that created the instrument.
     pub fn scope(&self) -> &InstrumentationScope {
         &self.scope
     }