[skywalking] 01/01: Update backend-fetcher.md

[ha...@apache.org]

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hanahmily pushed a commit to branch hanahmily-patch-1
in repository https://gitbox.apache.org/repos/asf/skywalking.git

commit 020f46d38d18c11f99ff48eaef9168c3aa8ae236
Author: Gao Hongtao <ha...@gmail.com>
AuthorDate: Tue Dec 8 16:00:27 2020 +0800

    Update backend-fetcher.md
---
 docs/en/setup/backend/backend-fetcher.md | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/docs/en/setup/backend/backend-fetcher.md b/docs/en/setup/backend/backend-fetcher.md
index 764e9b2..83918fc 100644
--- a/docs/en/setup/backend/backend-fetcher.md
+++ b/docs/en/setup/backend/backend-fetcher.md
@@ -3,11 +3,14 @@ Fetcher is a concept in SkyWalking backend. It uses pulling mode rather than [re
 read the data from the target systems. This mode is typically in some metrics SDKs, such as Prometheus.
 
 ## Prometheus Fetcher
+Suppose you want to enable `metric-custom.yaml` stored at `fetcher-prom-rules`, append its name to `enabledRules` of 
+ `promethues-fetcher` as below:
+ 
 ```yaml
 prometheus-fetcher:
-  selector: ${SW_PROMETHEUS_FETCHER:default}
+  selector: ${SW_PROMETHEUS_FETCHER:-}
   default:
-    active: ${SW_PROMETHEUS_FETCHER_ACTIVE:false}
+    enabledRules: ${SW_PROMETHEUS_FETCHER_ENABLED_RULES:"self,metric-custom"}
 ```
 
 ### Configuration file