can't get metrics from federate prometheus with consul

[顾鹏飞]

**What did you do?**

service discovery used consul

metrics_path: '/federate' in scrape_configs

**Environment**

* System information:

centos 7

* Prometheus version:

2.19.3

* Prometheus configuration file:

for example

```

scrape_configs:

      #The job name is added as a label `job=<job_name>` to any timeseries scraped from this config.

      - job_name: 'consul-m'

        honor_labels: true

        metrics_path: '/federate'

        consul_sd_configs:

        - server: '172.32.155.101:8500'

          services:

            - prometheus1

        relabel_configs:

        - regex: __meta_consul_service_metadata_(.+)

          action: labelmap

      - job_name: 'consul-f'

        honor_labels: true

        scheme: https

        tls_config:

          ca_file: /conf/pem/ca.pem

          cert_file: /conf/pem/client.pem

          key_file: /conf/pem/client-key.pem

          insecure_skip_verify: true

        metrics_path: '/federate'

        consul_sd_configs:

        - server: '172.32.155.101:8500'

          services:

            - prometheus2

        relabel_configs:

        - regex: __meta_consul_service_metadata_(.+)

          action: labelmap

```

* Logs:

```

no error logs

discovery services successfully and target state is up , but it can't get metrics ,it shows no data

curl get request '/api/v1/targets/metadata' return {"status":"success","data":[]}

```

[Brian Candler]

You can't just scrape the naked /federate endpoint, you need to give it a match[] query parameter (set using "params" in your scrape job).

I suggest you get it working with curl first, and then configure your scrape job accordingly:

curl -g 'http://prometheus:9090/federate'

=> empty response

curl -g 'http://prometheus:9090/federate?match[]={__name__="up"}'

=> response includes the given metrics

The documentation, with example configuration, is here: https://prometheus.io/docs/prometheus/latest/federation/#configuring-federation