docs: Optimize the sample configuration of prometheus job

[nic-6443]

Improved or typo fixed.

Now the sample configuration of prometheus job is as follows:

scrape_configs:
  - job_name: "apisix"
    metrics_path: "/apisix/prometheus/metrics"
    static_configs:
      - targets: ["127.0.0.1:9091"]

Such a configuration will inherit the scrape_interval in the global configuration of prometheus.
At the same time, the minimum time window of the rate function in the grafana dashboard provided is 30s.
sum(rate(apisix_bandwidth{instance=~"$instance"}[30s])) by (type)
Based on the above two points, if the scrape_interval in the user's prometheus global configuration is greater than 15s, the rate function may not calculate the result at some point in time, causing the dashboard to be unavailable.

The ingress nginx project has a similar PR: kubernetes/ingress-nginx#2884

Solution

Specify scrape_interval in the sample configuration

scrape_configs:
  - job_name: "apisix"
    scrape_interval: 15s
    metrics_path: "/apisix/prometheus/metrics"
    static_configs:
      - targets: ["127.0.0.1:9091"]

[spacewander]

The example of the documentation (including the grafana dashboard), is only an example. It doesn't mean you should do it like that. We can accept the PR, however, we should mention that the configuration is just an example.

[nic-6443]

The example of the documentation (including the grafana dashboard), is only an example. It doesn't mean you should do it like that. We can accept the PR, however, we should mention that the configuration is just an example.

Ok, add some comments after the scrape_interval configuration to illustrate the relationship with the rate function. Do you think it is ok?