Add detail about known error types to Kestrel connection instrument

Author: JamesNK

docs/core/diagnostics/built-in-metrics-aspnetcore.md

@@ -249,7 +249,7 @@ Available starting in: .NET 8.0.
 
 | Attribute  | Type | Description  | Examples  | Presence |
 |---|---|---|---|---|
-| `error.type` | string | The full name of exception type. | `System.OperationCanceledException`; `Contoso.MyException` | If an exception was thrown. |
+| `error.type` | string | Describes a type of error the connection ended with, or the unhandled exception type thrown during the connection pipeline. Known connection errors can be found [here](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-kestrel-metrics/#metric-kestrelconnectionduration). | `connection_reset`; `invalid_request_headers`; `System.OperationCanceledException` | If the connection ended with a known error or an exception was thrown. |
 | `network.protocol.name` | string | [OSI application layer](https://osi-model.com/application-layer/) or non-OSI equivalent. | `http`; `web_sockets` | Always |
 | `network.protocol.version` | string | Version of the protocol specified in `network.protocol.name`. | `1.1`; `2` | Always |
 | `network.transport` | string | [OSI transport layer](https://osi-model.com/transport-layer/) or [inter-process communication method](https://en.wikipedia.org/wiki/Inter-process_communication). | `tcp`; `unix` | Always |
@@ -260,6 +260,8 @@ Available starting in: .NET 8.0.
 
 As this metric is tracking the connection duration, and ideally http connections are used for multiple requests, the buckets should be longer than those used for request durations. For example, using [ 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 30, 60, 120, 300] provides an upper bucket of 5 mins.
 
+Starting in .NET 9.0, when a connection ends with a known error the `error.type` attribute value is set to the known error type. Known connection errors can be found [here](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-kestrel-metrics/#metric-kestrelconnectionduration).
+
 Available starting in: .NET 8.0.
 
 ##### Metric: `kestrel.rejected_connections`