Improved Kestrel Connection Metrics

We've made a significant improvement to Kestrel's connection metrics by including metadata about why a connection failed. The kestrel.connection.duration metric now includes the connection close reason in the error.type attribute.

Here is a small sample of the error.type values:

tls_handshake_failed - The connection requires TLS, and the TLS handshake failed.
connection_reset - The connection was unexpectedly closed by the client while requests were in progress.
request_headers_timeout - Kestrel closed the connection because it didn't receive request headers in time.
max_request_body_size_exceeded - Kestrel closed the connection because uploaded data exceeded max size.

Previously, diagnosing Kestrel connection issues required a server to record detailed, low-level logging. However, logs can be expensive to generate and store, and it can be difficult to find the right information among the noise.

Metrics are a much cheaper alternative that can be left on in a production environment with minimal impact. Collected metrics can drive dashboards and alerts. Once a problem is identified at a high-level with metrics, further investigation using logging and other tooling can begin.

We expect improved connection metrics to be useful in many scenarios:

Investigating performance issues caused by short connection lifetimes.
Observing ongoing external attacks on Kestrel that impact performance and stability.
Recording attempted external attacks on Kestrel that Kestrel's built-in security hardening prevented.

For more information, see ASP.NET Core metrics.