docs: [refinery] Add MaxConnectionAge default to values.yaml (#254)

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## Which problem is this PR solving?

The MaxConnectionAge default is 0 which leads to load balancers failing
to re-balance GRPC connections to Refinery. This means that one node in
a multi-node deployment receives all of the traffic and expends more
resources than it should redirecting all the traffic while the other
nodes sit idle.

This PR doesn't change the default but exposes it as a configuration
that can be set so operators can be in a better position to fix the
problem if they run into it.

## Short description of the changes

Adds some new  

## How to verify that this has the expected result

You can use the `helm template` command to see that the config is
present. When it comes to actually verifying the connections remain
open, I haven't cracked that nut yet.

charts/refinery/values.yaml

@@ -267,6 +267,19 @@ config:
     # By default that setting is 2GB, and this is set to 85% of that limit
     # 2 * 1024 * 1024 * 1024 * 0.80 = 1,717,986,918
     MaxAlloc: 1717986918
+
+  # Reflects: https://pkg.go.dev/google.golang.org/grpc/keepalive#ServerParameters
+  GRPCServerParameters:
+
+    # MaxConnectionAge is a duration for the maximum amount of time a
+    # connection may exist before it will be closed by sending a GoAway. A
+    # random jitter of +/-10% will be added to MaxConnectionAge to spread out
+    # connection storms.
+    # 0s sets duration to infinity which is the default:
+    # Recommended to set it to "3m" if there are multiple replicas
+    # https://github.com/grpc/grpc-go/blob/60a3a7e969c401ca16dbcd0108ad544fb35aa61c/internal/transport/http2_server.go#L220-L222
+    # Not eligible for live reload.
+    MaxConnectionAge: "0s"
 
   # Controls the parameters of the stress relief system. There is a metric called
   # stress_level that is emitted as part of refinery metrics. It is a measure of