fix(app/env): a lower default maximum per-host connection limit (#4005)

* chore(app/env): fix typo

Signed-off-by: katelyn martin <[email protected]>

* fix(app/env): a lower default maximum per-host connection limit

see also:
* #4004
* linkerd/linkerd2#14204

in #4004 we fixed an issue related to our HTTP/1.1 client's connection
pool.

this further hedges against future issues related to our HTTP client
exhausting resources available to its container. today, the limit by
default is `usize::MAX`, which is dramatically higher than the practical
limit.

this commit changes the limit for outbound idle connections per-host to
10,000.

Signed-off-by: katelyn martin <[email protected]>

---------

Signed-off-by: katelyn martin <[email protected]>

Author: cratelyn

linkerd/app/src/env.rs

@@ -343,12 +343,16 @@ const DEFAULT_INBOUND_HTTP1_CONNECTION_POOL_IDLE_TIMEOUT: Duration = Duration::f
 // TODO(ver) This should be configurable at the load balancer level.
 const DEFAULT_OUTBOUND_HTTP1_CONNECTION_POOL_IDLE_TIMEOUT: Duration = Duration::from_secs(3);
 
-// By default, we don't limit the number of connections a connection pol may
+// By default, we don't limit the number of connections a connection pool may
 // use, as doing so can severely impact CPU utilization for applications with
 // many concurrent requests. It's generally preferable to use the MAX_IDLE_AGE
 // limitations to quickly drop idle connections.
 const DEFAULT_INBOUND_MAX_IDLE_CONNS_PER_ENDPOINT: usize = usize::MAX;
-const DEFAULT_OUTBOUND_MAX_IDLE_CONNS_PER_ENDPOINT: usize = usize::MAX;
+// By default, we limit the number of outbound connections that may be opened
+// per-host. We pick a high number (10k) that shouldn't interfere with most
+// workloads, but will prevent issues with our outbound HTTP client from
+// exhausting the file descriptors available to the process.
+const DEFAULT_OUTBOUND_MAX_IDLE_CONNS_PER_ENDPOINT: usize = 10_000;
 
 // These settings limit the number of requests that have not received responses,
 // including those buffered in the proxy and dispatched to the destination