Document how to specify multiple excludeMetrics

The documentation mentions multiple patterns can be given by repeating
the `excludeMetrics` setting but it was unclear to me how to proceed.

The setting is fetched using a getStringList() which comes from jgit and
indeed one just need to repeat it. An example makes it clearer.

I also thought the matching was to be done against the metric names
exposed to Prometheus, but it is instead done against the Gerrit metric
name (`/` separated).

The doc mentions the matching is done on the whole key, document that it
is internally using `regex.Matcher.matches()` (and not `lookingAt()`
which matches at the start of the subject).  The multiple values
example does use a lead carret '^' which indicates matching at the start
of metric name has to be set explicitly.

Change-Id: I89fd2d4bb6523446ba54f741767336f907c800ac

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 070151f..6c41974 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -25,11 +25,29 @@
 for how to configure the integration with Prometheus.
 
 plugin.@PLUGIN@.excludeMetrics
-:   Regex pattern used to exclude metrics from the report. It can be specified multiple times.
-    Note that pattern matching is done on the whole metric name, not only on a part of it.
-    By default no metric is excluded.
-    For example, to exclude all cache metrics, use: `excludeMetrics = cache.*`
+:   Regex pattern used to exclude metrics from the report.
 
+	The matching is done against the Gerrit metrics names as documented on
+	['Metrics'](@URL@Documentation/metrics.html). The pattern matching is done
+	internally using `regex.Matcher.matches()` and can match anywhere (not
+    necessarly starting at the beginning of the metric name)
+
+    By default no metric is excluded.
+
+	For examples:
+
+	To exclude all metrics matching `cache` at some place in their key, use:
+    `excludeMetrics = cache.*`
+
+    To exclude multiple metrics:
+	```
+    [plugin "metrics-reporter-prometheus"]
+        excludeMetrics = ^http/server/rest_api/.*
+        excludeMetrics = ^license/cla_check_count
+        excludeMetrics = ^plugin/latency/.*
+        excludeMetrics = ^reviewer_suggestion/.*
+        excludeMetrics = ^sequence/next_id_latency.*
+    ```
 
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