Record selected egress region in metrics

- Helps diagnose server selection/capacity issues.

psiphon/server/api.go

@@ -592,6 +592,7 @@ var baseRequestParams = append(
 		{"padding", isAnyString, requestParamOptional | requestParamLogStringLengthAsInt},
 		{"pad_response", isIntString, requestParamOptional | requestParamLogStringAsInt},
 		{"is_replay", isBooleanFlag, requestParamOptional | requestParamLogFlagAsBool},
+		{"egress_region", isRegionCode, requestParamOptional},
 	},
 	upstreamFragmentorParams...)
 

psiphon/serverApi.go

@@ -805,6 +805,10 @@ func getBaseAPIParameters(
 	}
 	params["is_replay"] = isReplay
 
+	if config.EgressRegion != "" {
+		params["egress_region"] = config.EgressRegion
+	}
+
 	if dialParams.DialConnMetrics != nil {
 		metrics := dialParams.DialConnMetrics.GetMetrics()
 		for name, value := range metrics {