Simplify the Getting Started documentation. (#7193)

- Reduce the level of entry to start gathering metrics with prometheus
  by suggesting to just download pre-built exporters instead of requiring
  the reader to download an entire Golang build chain and checkout a project.

Fix #6956

Signed-off-by: Harold Dost <h.dost@criteo.com>

docs/getting_started.md

@@ -118,7 +118,7 @@ For more about the expression language, see the
 To graph expressions, navigate to http://localhost:9090/graph and use the "Graph"
 tab.
 
-For example, enter the following expression to graph the per-second rate of chunks 
+For example, enter the following expression to graph the per-second rate of chunks
 being created in the self-scraped Prometheus:
 
 ```
@@ -132,36 +132,26 @@ Experiment with the graph range parameters and other settings.
 Let us make this more interesting and start some example targets for Prometheus
 to scrape.
 
-The Go client library includes an example which exports fictional RPC latencies
-for three services with different latency distributions.
-
-Ensure you have the [Go compiler installed](https://golang.org/doc/install) and
-have a [working Go build environment](https://golang.org/doc/code.html) (with
-correct `GOPATH`) set up.
-
-Download the Go client library for Prometheus and run three of these example
-processes:
+The Node Exporter is used as an example target, for more information on using it
+[see these instructions.](https://prometheus.io/docs/guides/node-exporter/)
 
 ```bash
-# Fetch the client library code and compile example.
-git clone https://github.com/prometheus/client_golang.git
-cd client_golang/examples/random
-go get -d
-go build
+tar -xzvf node_exporter-*.*.tar.gz
+cd node_exporter-*.*
 
 # Start 3 example targets in separate terminals:
-./random -listen-address=:8080
-./random -listen-address=:8081
-./random -listen-address=:8082
+./node_exporter --web.listen-address 127.0.0.1:8080
+./node_exporter --web.listen-address 127.0.0.1:8081
+./node_exporter --web.listen-address 127.0.0.1:8082
 ```
 
 You should now have example targets listening on http://localhost:8080/metrics,
 http://localhost:8081/metrics, and http://localhost:8082/metrics.
 
-## Configuring Prometheus to monitor the sample targets
+## Configure Prometheus to monitor the sample targets
 
 Now we will configure Prometheus to scrape these new targets. Let's group all
-three endpoints into one job called `example-random`. However, imagine that the
+three endpoints into one job called `node`. However, imagine that the
 first two endpoints are production targets, while the third one represents a
 canary instance. To model this in Prometheus, we can add several groups of
 endpoints to a single job, adding extra labels to each group of targets. In
@@ -173,7 +163,7 @@ section in your `prometheus.yml` and restart your Prometheus instance:
 
 ```yaml
 scrape_configs:
-  - job_name:       'example-random'
+  - job_name:       'node'
 
     # Override the global default and scrape targets from this job every 5 seconds.
     scrape_interval: 5s
@@ -189,8 +179,7 @@ scrape_configs:
 ```
 
 Go to the expression browser and verify that Prometheus now has information
-about time series that these example endpoints expose, such as the
-`rpc_durations_seconds` metric.
+about time series that these example endpoints expose, such as `node_cpu_seconds_total`.
 
 ## Configure rules for aggregating scraped data into new time series
 
@@ -198,27 +187,26 @@ Though not a problem in our example, queries that aggregate over thousands of
 time series can get slow when computed ad-hoc. To make this more efficient,
 Prometheus allows you to prerecord expressions into completely new persisted
 time series via configured recording rules. Let's say we are interested in
-recording the per-second rate of example RPCs
-(`rpc_durations_seconds_count`) averaged over all instances (but
-preserving the `job` and `service` dimensions) as measured over a window of 5
-minutes. We could write this as:
+recording the per-second rate of cpu time (`node_cpu_seconds_total`) averaged
+over all cpus per instance (but preserving the `job`, `instance` and `mode`
+dimensions) as measured over a window of 5 minutes. We could write this as:
 
 ```
-avg(rate(rpc_durations_seconds_count[5m])) by (job, service)
+avg by (job, instance, mode) (rate(node_cpu_seconds_total[5m]))
 ```
 
 Try graphing this expression.
 
 To record the time series resulting from this expression into a new metric
-called `job_service:rpc_durations_seconds_count:avg_rate5m`, create a file
+called `job_instance_mode:node_cpu_seconds:avg_rate5m`, create a file
 with the following recording rule and save it as `prometheus.rules.yml`:
 
 ```
 groups:
-- name: example
+- name: cpu-node
   rules:
-  - record: job_service:rpc_durations_seconds_count:avg_rate5m
-    expr: avg(rate(rpc_durations_seconds_count[5m])) by (job, service)
+  - record: job_instance_mode:node_cpu_seconds:avg_rate5m
+    expr: avg by (job, instance, mode) (rate(node_cpu_seconds_total[5m]))
 ```
 
 To make Prometheus pick up this new rule, add a `rule_files` statement in your `prometheus.yml`. The config should now
@@ -245,7 +233,7 @@ scrape_configs:
     static_configs:
       - targets: ['localhost:9090']
 
-  - job_name:       'example-random'
+  - job_name:       'node'
 
     # Override the global default and scrape targets from this job every 5 seconds.
     scrape_interval: 5s
@@ -261,5 +249,5 @@ scrape_configs:
 ```
 
 Restart Prometheus with the new configuration and verify that a new time series
-with the metric name `job_service:rpc_durations_seconds_count:avg_rate5m`
+with the metric name `job_instance_mode:node_cpu_seconds:avg_rate5m`
 is now available by querying it through the expression browser or graphing it.