Add detail about known error types to Kestrel connection metric (#43584)

Co-authored-by: Genevieve Warren <[email protected]>

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 at [Semantic Conventions for Kestrel web server metrics](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-kestrel-metrics/). | `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 |
@@ -260,7 +260,9 @@ 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.
 
-Available starting in: .NET 8.0.
+Starting in .NET 9, 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 at [Semantic Conventions for Kestrel web server metrics](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-kestrel-metrics/).
+
+Available starting in: .NET 8.
 
 ##### Metric: `kestrel.rejected_connections`