Document the difference between scrapemodes

docs/metrics.md

@@ -82,8 +82,13 @@ kminion_kafka_consumer_group_topic_partition_lag{group_id="bigquery-sink",partit
 # HELP kminion_kafka_consumer_group_topic_lag The number of messages a consumer group is lagging behind across all partitions in a topic
 # TYPE kminion_kafka_consumer_group_topic_lag gauge
 kminion_kafka_consumer_group_topic_lag{group_id="bigquery-sink",topic_name="shop-activity"} 147481
+```
 
+#### Offset Commits Metrics
+The following metrics are only available when KMinion is configured to use `scrapeMode: offsetsTopic`.
+```
 # HELP kminion_kafka_consumer_group_offset_commits_total The number of offsets committed by a group
 # TYPE kminion_kafka_consumer_group_offset_commits_total counter
 kminion_kafka_consumer_group_offset_commits_total{group_id="bigquery-sink"} 1098
 ```
+

docs/reference-config.yaml

@@ -52,6 +52,12 @@ minion:
     enabled: true
     # Mode specifies whether we export consumer group offsets using the Admin API or by consuming the internal
     # __consumer_offsets topic. Both modes have their advantages and disadvantages.
+    # * adminApi:
+    #     - Useful for managed kafka clusters that do not provide access to the offsets topic.
+    # * offsetsTopic
+    #     - Enables kminion_kafka_consumer_group_offset_commits_total metrics.
+    #     - Processing the offsetsTopic requires slightly more memory than using the adminApi. The amount depends on the
+    #       size and throughput of the offsets topic.
     scrapeMode: adminApi # Valid values: adminApi, offsetsTopic
     # Granularity can be per topic or per partition. If you want to reduce the number of exported metric series and
     # you aren't interested in per partition lags you could choose "topic" where all partition lags will be summed