dekaf: Track latest committed offset by group and journal

Looks like this:

```
dekaf_committed_offset{group_id="joseph-dekaf-testing-1100-1",journal_name="demo/bots-humans-edits/pivot=00"} 73173569244
dekaf_committed_offset{group_id="joseph-dekaf-testing-1100-0",journal_name="demo/wikipedia/recentchange/pivot=00"} 2036875838205
```

crates/dekaf/src/session.rs

@@ -847,6 +847,14 @@ impl Session {
             .connect_to_group_coordinator(req.group_id.as_str())
             .await?;
 
+        let flow_client = self
+            .auth
+            .as_mut()
+            .ok_or(anyhow::anyhow!("Session not authenticated"))?
+            .authenticated_client()
+            .await?
+            .clone();
+
         client
             .ensure_topics(
                 mutated_req
@@ -861,6 +869,42 @@ impl Session {
 
         for topic in resp.topics.iter_mut() {
             topic.name = self.decrypt_topic_name(topic.name.to_owned());
+
+            let collection_partitions = Collection::new(&flow_client, topic.name.as_str())
+                .await?
+                .context(format!("unable to look up partitions for {:?}", topic.name))?
+                .partitions;
+
+            for partition in &topic.partitions {
+                if let Some(error) = partition.error_code.err() {
+                    tracing::warn!(topic=?topic.name,partition=partition.partition_index,?error,"Got error from upstream Kafka when trying to commit offsets");
+                } else {
+                    let journal_name = collection_partitions
+                        .get(partition.partition_index as usize)
+                        .context(format!(
+                            "unable to find partition {} in collection {:?}",
+                            partition.partition_index, topic.name
+                        ))?
+                        .spec
+                        .name
+                        .to_owned();
+
+                    let committed_offset = req
+                        .topics
+                        .iter()
+                        .find(|req_topic| req_topic.name == topic.name)
+                        .context(format!("unable to find topic in request {:?}", topic.name))?
+                        .partitions
+                        .get(partition.partition_index as usize)
+                        .context(format!(
+                            "unable to find partition {}",
+                            partition.partition_index
+                        ))?
+                        .committed_offset;
+
+                    metrics::gauge!("dekaf_committed_offset", "group_id"=>req.group_id.to_string(),"journal_name"=>journal_name).set(committed_offset as f64);
+                }
+            }
         }
 
         Ok(resp)