support disable keep alive http connection (#982)

* Support disable keep http connection alive
Signed-off-by: penghuazhou <1021556870@qq.com>

* point users here to env KEDA_HTTP_DISABLE_KEEP_ALIVE in Kubernetes Deployments
Signed-off-by: penghuazhou <1021556870@qq.com>

* change variables to variable
Signed-off-by: penghuazhou <1021556870@qq.com>

content/docs/2.9/operate/cluster.md

@@ -75,6 +75,19 @@ $env:KEDA_HTTP_DEFAULT_TIMEOUT=1000
 
 All applicable scalers will use this timeout. Setting a per-scaler timeout is currently unsupported.
 
+## HTTP connection disable keep alive
+
+Keep alive behaviour is enabled by default for every HTTP connection, this could stack a huge amount of connections (one per scaler) in some scenarios. 
+
+You can disable keep alive for every HTTP connection by adding the relevant environment variable to both the KEDA Operator, and KEDA Metrics Server deployments:
+
+```yaml
+- env:
+    KEDA_HTTP_DISABLE_KEEP_ALIVE: true
+```
+
+All applicable scalers will use this keep alive behaviour. Setting a per-scaler keep alive behaviour is currently unsupported.
+
 ## HTTP Proxies
 
 Some scalers issue HTTP requests to external servers (i.e. cloud services). As certain companies require external servers to be accessed by proxy servers, adding the relevant environment variables to both the KEDA Operator, and KEDA Metrics Server deployments (HTTP_PROXY, HTTPS_PROXY, NO_PROXY, etc.) would allow the scaler to connect via the desired proxy.