OM2: Add unknown metadata example (#2937)

* OM2: Add unknown metadata example

Signed-off-by: bwplotka <bwplotka@gmail.com>

* Update docs/specs/om/open_metrics_spec_2_0.md

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

---------

Signed-off-by: bwplotka <bwplotka@gmail.com>
Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Author: bwplotka

docs/specs/om/open_metrics_spec_2_0.md

@@ -705,7 +705,7 @@ If a unit is specified it MUST be provided in a UNIT metadata line. In addition,
 
 Be aware that exposing metrics without the unit being a suffix (or infix) of the MetricFamily name directly to end-users may reduce the usability due to confusion about what the metric's unit is.
 
-A valid example for a foo_seconds metric with a unit of "seconds":
+A valid example for a `foo_seconds_total` metric with a unit of "seconds":
 
 ```openmetrics-add-eof
 # TYPE foo_seconds_total counter
@@ -743,6 +743,19 @@ There MUST NOT be more than one of each type of metadata line for a MetricFamily
 
 Aside from this metadata and the EOF line at the end of the message, you MUST NOT expose lines beginning with a #.
 
+##### Unknown metadata
+
+Ingestors MUST support Metric Families without metadata lines.
+
+A valid, but discouraged example, for `foo_seconds_total` counter and a set of unrelated, unknown type metrics without metadata lines:
+
+```openmetrics-add-eof
+# TYPE foo_seconds_total counter
+foo_seconds_total 1
+foo_milliseconds_total 2
+foo_count 3
+```
+
 #### Metric
 
 Metrics MUST NOT be interleaved. See the StateSet example below.