docs: clarify the configuration of the Prometheus and Opentelemetry p…

…lugins in standalone mode (#11396)

* aligned the references to config/config.yaml

* replaced the misleading heading API with Metrics endpoint

Users reading the documentation can easily get confused by the term "API". When running in standalone mode, the "API" is disabled, leading to the misconception that the Prometheus endpoint cannot be exposed.

As the Prometheus metrics endpoint is not part of APISIX's API, this confusion can be avoided.

docs/en/latest/plugins/opentelemetry.md

@@ -74,7 +74,7 @@ For instance, a possible scenario occurs when the plugin attribute `trace_id_sou
 
 :::
 
-You can configure these as shown below:
+You can configure these as shown below in your configuration file (`conf/config.yaml`):
 
 ```yaml title="conf/config.yaml"
 plugin_attr:
@@ -105,7 +105,7 @@ The following nginx variables are set by OpenTelemetry:
 
 How to use variables? you have to add it to your configuration file (`conf/config.yaml`):
 
-```yaml title="./conf/config.yaml"
+```yaml title="conf/config.yaml"
 http:
     enable_access_log: true
     access_log: "/dev/stdout"

docs/en/latest/plugins/prometheus.md

@@ -96,9 +96,9 @@ plugin_attr:
       - 505
 ```
 
-## API
+## Metrics endpoint
 
-This Plugin will add the API endpoint `/apisix/prometheus/metrics` or your custom export URI for exposing the metrics.
+This Plugin will add the metrics endpoint `/apisix/prometheus/metrics` or your custom export URI for exposing the metrics.
 
 These metrics are exposed by a separate Prometheus server address. By default, the address is `127.0.0.1:9091`. You can change it in your configuration file (`conf/config.yaml`):