Add Prometheus statistics and an example to showcase them using Grafana

examples/grafana/README.md

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+# Display runtime metrics using Prometheus and Grafana
+
+The Fn server exports metrics using [Prometheus](https://prometheus.io/). This allows [Grafana](https://grafana.com/) to be used to display these metrics graphically. 
+
+-- screenshot
+
+## Start a fn server and deploy some functions
+
+This example requires a Fn server to be running and that you have deployed one or more functions. 
+See the [front page](/README.md) or any of the other examples for instructions. 
+
+The steps below assume that the Fn server is running at `localhost:8080`.
+
+## Examine the endpoint used to export metrics to Prometheus
+
+The Fn server exports metrics to Prometheus using the API endpoint `/metrics`. 
+
+Try pointing your browser at [http://localhost:8080/metrics](http://localhost:8080/metrics).
+This will display the metrics in prometheus format.
+
+## Start Prometheus
+
+Open a terminal window and navigate to the directory containing this example.
+
+Examine the provised Prometheus configuration file:
+
+```
+cat prometheus.yml
+```
+
+This gives
+
+``` yml
+global:
+  scrape_interval:     15s # By default, scrape targets every 15 seconds.
+
+  # Attach these labels to any time series or alerts when communicating with
+  # external systems (federation, remote storage, Alertmanager).
+  external_labels:
+    monitor: 'fn-monitor'
+
+# A scrape configuration containing exactly one endpoint to scrape:
+# Here it's the Fn server
+scrape_configs:
+  # The job name is added as a label `job=<job_name>` to any timeseries scraped from this config.
+  - job_name: 'functions'
+
+    # Override the global default and scrape targets from this job every 5 seconds.
+    scrape_interval: 5s
+
+    static_configs:
+      # Specify all the fn servers from which metrics will be scraped
+      - targets: ['localhost:8080'] # Uses /metrics by default
+```
+Note the last line. This specifies the host and port of the Fn server from which metrics will be obtained. 
+If you are running a cluster of Fn servers then you can specify them all here.
+
+Now start Prometheus, specifying this config file:
+```
+docker run --name=prometheus -d -p 9090:9090 \
+  --mount type=bind,source=`pwd`/prometheus.yml,target=/etc/prometheus/prometheus.yml \
+  --add-host="localhost:`route | grep default | awk '{print $2}'`" prom/prometheus
+```
+Note: The parameter `` --add-host="localhost:`route | grep default | awk '{print $2}'`" `` means that Prometheus can use localhost to refer to the host. (The expression `` `route | grep default | awk '{print $2}'` ``  returns the IP of the host).
+
+Open a browser on Prometheus's graph tool at [http://localhost:9090/graph](http://localhost:9090/graph). If you wish you can use this to view metrics and display metrics from the fn server: see the [Prometheus](https://prometheus.io/) documentation for instructions. Alternatively continue with the next step to view a ready-made set of graphs in Grafana.
+
+## Start Grafana and load the example dashboard
+
+[Grafana](https://grafana.com/) provides powerful and flexible facilities to create graphs of any metric available to Prometheus. This example provides a ready-made dashboard that displays the numbers of functions that are queued, running, completed and failed. 
+
+Open a terminal window and navigate to the directory containing this example.
+
+Start Grafana on port 3000:
+```
+docker run --name=grafana -d -p 3000:3000 \
+  --add-host="localhost:`route | grep default | awk '{print $2}'`" grafana/grafana
+```
+
+Open a browser on Grafana at [http://localhost:3000](http://localhost:3000).
+
+Login using the default user `admin` and default password `admin`.
+
+Create a datasource to obtain metrics from Promethesus:
+* Click on **Add data source**. In the form that opens:
+* Set **Name** to `PromDS` (or whatever name you choose)
+* Set **Type** to `Prometheus`
+* Set **URL** to `http://localhost:9090` 
+* Set **Access** to `proxy`
+* Click **Add** and then **Save and test**
+
+Import the example dashboard that displays metrics from the Fn server:
+* Click on the main menu at the top left and choose **Dashboards** and then **Home**
+* Click on **Home** at the top and then **Import dashboard**
+* In the dialog that opens, click **Upload .json file** and specify `fn_grafana_dashboard.json` in this example's directory.
+* Specify the Prometheus data source that you just created
+* Click **Import**
+
+You should then see the dashboard shown above. Now execute some functions and see the graphs update.
+

examples/grafana/prometheus.yml

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+global:
+  scrape_interval:     15s # By default, scrape targets every 15 seconds.
+
+  # Attach these labels to any time series or alerts when communicating with
+  # external systems (federation, remote storage, Alertmanager).
+  external_labels:
+    monitor: 'fn-monitor'
+
+# A scrape configuration containing exactly one endpoint to scrape:
+# Here it's the Fn server
+scrape_configs:
+  # The job name is added as a label `job=<job_name>` to any timeseries scraped from this config.
+  - job_name: 'functions'
+
+    # Override the global default and scrape targets from this job every 5 seconds.
+    scrape_interval: 5s
+
+    static_configs:
+      # Specify all the fn servers from which metrics will be scraped
+      - targets: ['localhost:8080'] # Uses /metrics by default