added caddyfile support for the dynamic srv's grace_period option

caddytest/integration/caddyfile_adapt/reverse_proxy_dynamic_upstreams_grace_period.caddyfiletest

@@ -0,0 +1,38 @@
+:8884 {
+	reverse_proxy {
+		dynamic srv {
+			name foo
+			refresh 5m
+			grace_period 5s
+		}
+	}
+}
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":8884"
+					],
+					"routes": [
+						{
+							"handle": [
+								{
+									"dynamic_upstreams": {
+										"grace_period": 5000000000,
+										"name": "foo",
+										"refresh": 300000000000,
+										"source": "srv"
+									},
+									"handler": "reverse_proxy"
+								}
+							]
+						}
+					]
+				}
+			}
+		}
+	}
+}

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -1356,6 +1356,7 @@ func (h *CopyResponseHeadersHandler) UnmarshalCaddyfile(d *caddyfile.Dispenser)
 //	    resolvers           <resolvers...>
 //	    dial_timeout        <timeout>
 //	    dial_fallback_delay <timeout>
+//      grace_period        <duration>
 //	}
 func (u *SRVUpstreams) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 	d.Next() // consume upstream source name
@@ -1435,7 +1436,15 @@ func (u *SRVUpstreams) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				return d.Errf("bad delay value '%s': %v", d.Val(), err)
 			}
 			u.FallbackDelay = caddy.Duration(dur)
-
+		case "grace_period":
+			if !d.NextArg() {
+				return d.ArgErr()
+			}
+			dur, err := caddy.ParseDuration(d.Val())
+			if err != nil {
+				return d.Errf("bad grace period value '%s': %v", d.Val(), err)
+			}
+			u.GracePeriod = caddy.Duration(dur)
 		default:
 			return d.Errf("unrecognized srv option '%s'", d.Val())
 		}