rpc: disable use of http proxies by default

GRPC uses the HTTPS_PROXY environment variable by default[1]. This is
surprising, and likely undesirable for CRDB because it turns the proxy
into an availability risk and a throughput bottleneck. We disable the
use of proxies by default when retrieving grpc dial options.

This diff came up in the context of cockroachdb#55289, and was a recent regression
in 20.2 after having introduced the join rpc in cockroachdb#52526 (it's the rpc
responsible for adding new nodes to the cluster cockroachdb#52526). Our existing
RPC connections use the `WithContextDialer` option already, which has the
side effect of disabling proxy use. The join RPC didn't, so an existing
cluster upgrade to 20.2 wouldn't use the proxy until a node was added.

Fixes cockroachdb#55289. This will need to get backported to release-20.2.

[1]: https://github.com/grpc/grpc-go/blob/c0736608/Documentation/proxy.md

Release note (bug fix): Previously we used the HTTPS_PROXY variable for
the "join RPC" when adding a node to the cluster (the RPC prevents new
clusters from starting or adding nodes to an existing cluster). The
proxy needed to configured to transparently pass HTTP/2+GRPC inter-node
traffic. This was an unintentional addition, and this patch ignores the
proxies for all intra-node traffic. They were already ignored in
releases prior to 20.2.

pkg/rpc/context.go

@@ -733,6 +733,14 @@ func (ctx *Context) grpcDialOptions(
 		dialOpts = append(dialOpts, grpc.WithDefaultCallOptions(grpc.UseCompressor((snappyCompressor{}).Name())))
 	}
 
+	// GRPC uses the HTTPS_PROXY environment variable by default[1]. This is
+	// surprising, and likely undesirable for CRDB because it turns the proxy
+	// into an availability risk and a throughput bottleneck. We disable the use
+	// of proxies by default.
+	//
+	// [1]: https://github.com/grpc/grpc-go/blob/c0736608/Documentation/proxy.md
+	dialOpts = append(dialOpts, grpc.WithNoProxy())
+
 	var unaryInterceptors []grpc.UnaryClientInterceptor
 
 	if tracer := ctx.AmbientCtx.Tracer; tracer != nil {