Do not close health check connections when draining

Description: When Envoy is draining (via /healthcheck/fail) the HttpConnectionManager will send a GOAWAY or Connection:close on both application and health check connections – even if the health check requests are setting Connection: keep-alive.

In an active health checking setup, this causes unnecessary connection churn for health check connections – clients will continue to send health check requests, but will need to open a new connection each time.

This is problematic for servers that (a) receive a lot of health checks (b) use mTLS even for health check requests.

I would expect that if the health check filter is configured, Envoy would keep these connections alive.

Repro steps:

• Configure Envoy as below
• Drain Envoy with healthcheck/fail
• Send HTTP/1 health check request with Connection: keep-alive
• Observe 503 response with Connection: close header

Config:

• HttpConnectionManager
• HTTP Health Check filter

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If this behaviour seems reasonable to folks, would love to hear how to approach adding this to Envoy.