Create EXAMPLES.md (#63)

* Create EXAMPLES.md

Add examples how to use the Fortigate Exporter

EXAMPLES.md

@@ -0,0 +1,67 @@
+# PromQL usage examples for Fortigate Exporter
+
+PromQL is an advanced language and proper usage of it requires firstly to understand
+the language itself and secondly how the metrics are organized.
+
+To help the users of this exporter, here are some examples to illustrate
+how one could use the power of PromQL to query interesting facts about one's
+environment.
+
+For an introduction to the PromQL basics, see the official Prometheus
+[documentation](https://prometheus.io/docs/prometheus/latest/querying/examples/).
+
+## Most Active Firewall Policies
+
+Using the `topk` function it is easy to get the most active firewall policies
+based on whatever ranking function you want.
+
+Example: `topk(3, rate(fortigate_policy_bytes_total[15m])) * 8`
+
+This will return the top 3 most active policies based upon the bytes transfered on
+average the last 15m. The output will be bits/s.
+
+| Element | Value| 
+|---------|------|
+`{id="9",instance="fgt-a",job="fortigate",name="SSH",protocol="ipv6",uuid="5cd4b62e-4904-51eb-b4a9-f52e75461e52",vdom="bluecmd"}` |	102805.48826815643
+`{id="28",instance="fgt-a",job="fortigate",name="fortigate exporter",protocol="ipv6",uuid="8753dcd2-4a07-51eb-bf78-c61aa31a8e1e",vdom="bluecmd"}` |	34443.977653631286
+`{id="11",instance="fgt-a",job="fortigate",name="tera cluster mgmt",protocol="ipv4",uuid="19a2c192-4905-51eb-9d14-5d1249566588",vdom="bluecmd"}` | 15239.401117318435
+
+## Adding Version Information
+
+Using `group_left` one can add data from other metadata metrics like `fortigate_version_info`.
+
+Example: `fortigate_memory_usage_ratio * on (instance) group_left (version) fortigate_version_info`
+
+| Element | Value| 
+|---------|------|
+`{instance="fgt-test",job="fortigate",version="v6.4.5"}`	| 0.12
+`{instance="fgt-a",job="fortigate",version="v6.4.4"}`	| 0.23
+`{instance="fgt-b",job="fortigate",version="v6.4.4"}`	| 0.16
+
+## Expiring Certificates
+
+Dealing with certificates can unfortunately be quite complex.
+However, by using PromQL it is possible to construct a query that returns
+all certificates expiring within the coming 90 days.
+
+Example:
+
+```
+floor( # return whole days
+  (
+    (
+      fortigate_certificate_valid_to_seconds and 
+      fortigate_certificate_cmdb_references > 0 and  # only include certificates that are used for something
+      on (instance,name,vdom) fortigate_certificate_info{status="valid"}  # we do not care about things like CSRs
+    )
+    - time()
+  ) / 86400 # convert seconds to days
+)
+< 90 # number of days to filter for
+```
+
+| Element | Value| 
+|---------|------|
+`{instance="fgt-test",job="fortigate",name="LetsEncrypt-2021-03-13",scope="global",source="user",vdom="root"}` |	89
+`{instance="fgt-a",job="fortigate",name="LetsEncrypt-2021-01-05",scope="global",source="user",vdom="root"}` |	21
+`{instance="fgt-b",job="fortigate",name="LetsEncrypt-2021-01-05",scope="global",source="user",vdom="root"}` |	21

README.md

@@ -11,6 +11,8 @@ Prometheus exporter for Fortigate firewalls.
 Right now the exporter supports a quite limited set of metrics, but it is very easy to add!
 Open an issue if your favorite metric is missing.
 
+For example PromQL usage, see [EXAMPLES](EXAMPLES.md).
+
 Supported metrics right now as follows.
 
 Global: