Add sample term to glossary

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

content/docs/concepts/jobs_instances.md

@@ -29,8 +29,8 @@ depends on the `honor_labels` configuration option. See the
 [scrape configuration documentation](/docs/operating/configuration/#%3Cscrape_config%3E)
 for more information.
 
-For each instance scrape, Prometheus stores a sample in the following
-time series:
+For each instance scrape, Prometheus stores a [sample](/docs/introduction/glossary#sample) in
+the following time series:
 
 * `up{job="<job-name>", instance="<instance-id>"}`: `1` if the instance is
    healthy, i.e. reachable, or `0` if the scrape failed.

content/docs/introduction/glossary.md

@@ -108,6 +108,13 @@ write into a format the other system can understand.
 
 A remote write endpoint is what Prometheus talks to when doing a remote write.
 
+### Sample
+
+A sample is the smallest unit of time series data in Prometheus. Each sample consists of:
+
+* A float64 value
+* A millisecond-precision timestamp
+
 ### Silence
 
 A silence in the Alertmanager prevents alerts, with labels matching the silence, from