ENG-48179: setting default scheme to passthrough

With NewClient API usage, we are facing issues at few customers
who have intermediate proxies between collector and platform.
With NewClient API instead DialContext, DNS resolution happens
on the client side while it should happen on proxy.
Also, with SGProxy client does not get the correct certificate.
Passthrough scheme was the prior default and prevents resolution to
happen beforehand. This change can be removed once grpc fixes grpc/grpc-go#7556
and otel collector picks the fix

config/configgrpc/configgrpc.go

@@ -8,6 +8,7 @@ import (
 	"crypto/tls"
 	"errors"
 	"fmt"
+	"google.golang.org/grpc/resolver"
 	"strings"
 	"time"
 
@@ -224,6 +225,7 @@ func (gcs *ClientConfig) ToClientConn(ctx context.Context, host component.Host,
 		return nil, err
 	}
 	opts = append(opts, extraOpts...)
+	resolver.SetDefaultScheme("passthrough")
 	return grpc.NewClient(gcs.sanitizedEndpoint(), opts...)
 }