dovecot: Use group_by, not fields for metric labels

As the former does not do exactly what it says it does in documentation.
Also, we decrease the default scrape interval for Prometheus from 1m to
30s to improve granularity of data.

service/dovecot/container/config/conf.d/10-metrics.conf

@@ -7,12 +7,12 @@ service stats {
 # Authentication metrics.
 metric auth_passdb_request {
     filter = event=auth_passdb_request_finished
-    fields = passdb_name result
+    group_by = passdb_name result
 }
 
 metric auth_userdb_request {
     filter = event=auth_userdb_request_finished
-    fields = userdb_name result
+    group_by = userdb_name result
 }
 
 # Connection metrics.
@@ -27,28 +27,28 @@ metric server_connection {
 # HTTP client metrics.
 metric http_client_request {
     filter = event=http_request_finished
-    fields = status_code
+    group_by = status_code
 }
 
 # IMAP command metrics.
 metric imap_command {
     filter = event=imap_command_finished
-    fields = user cmd_name tagged_reply_state
+    group_by = user cmd_name tagged_reply_state
 }
 
 # SQL metrics.
 metric sql_query {
     filter = event=sql_query_finished
-    fields = query error_code
+    group_by = query error_code
 }
 
 # Sieve metrics.
 metric sieve_script {
     filter = event=sieve_runtime_script_finished
-    fields = script_name error
+    group_by = script_name error
 }
 
 metric sieve_action {
     filter = event=sieve_action_finished
-    fields = action_name
+    group_by = action_name
 }

service/prometheus/container/config/prometheus.yml

@@ -1,3 +1,5 @@
+global:
+  scrape_interval: 30s
 scrape_configs:
   - job_name: prometheus
     honor_labels: true