Show more processes in pg_stat_activity.

Previously, auxiliary processes and background workers not connected
to a database (such as the logical replication launcher) weren't
shown.  Include them, so that we can see the associated wait state
information.  Add a new column to identify the processes type, so that
people can filter them out easily using SQL if they wish.

Before this patch was written, there was discussion about whether we
should expose this information in a separate view, so as to avoid
contaminating pg_stat_activity with things people might not want to
see.  But putting everything in pg_stat_activity was a more popular
choice, so that's what the patch does.

Kuntal Ghosh, reviewed by Amit Langote and Michael Paquier.  Some
revisions and bug fixes by me.

Discussion: http://postgr.es/m/CA+TgmoYES5nhkEGw9nZXU8_FhA8XEm8NTm3-SO+3ML1B81Hkww@mail.gmail.com

Author: robertmhaas

doc/src/sgml/monitoring.sgml

@@ -620,8 +620,8 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
     <row>
      <entry><structfield>backend_start</></entry>
      <entry><type>timestamp with time zone</></entry>
-     <entry>Time when this process was started, i.e., when the
-      client connected to the server
+     <entry>Time when this process was started.  For client backends,
+      this is the time the client connected to the server.
      </entry>
     </row>
     <row>
@@ -797,6 +797,17 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <xref linkend="guc-track-activity-query-size">.
      </entry>
     </row>
+    <row>
+     <entry><structfield>backend_type</structfield></entry>
+     <entry><type>text</type></entry>
+     <entry>Type of current backend. Possible types are 
+      <literal>autovacuum launcher</>, <literal>autovacuum worker</>,
+      <literal>background worker</>, <literal>background writer</>,
+      <literal>client backend</>, <literal>checkpointer</>,
+      <literal>startup</>, <literal>walreceiver</>,
+      <literal>walsender</> and <literal>walwriter</>.
+     </entry>
+    </row>
    </tbody>
    </tgroup>
   </table>

src/backend/bootstrap/bootstrap.c

@@ -387,6 +387,10 @@ AuxiliaryProcessMain(int argc, char *argv[])
 		/* finish setting up bufmgr.c */
 		InitBufferPoolBackend();
 
+		/* Initialize backend status information */
+		pgstat_initialize();
+		pgstat_bestart();
+
 		/* register a before-shutdown callback for LWLock cleanup */
 		before_shmem_exit(ShutdownAuxiliaryProcess, 0);
 	}

src/backend/catalog/system_views.sql

@@ -716,7 +716,8 @@ CREATE VIEW pg_stat_activity AS
             S.state,
             S.backend_xid,
             s.backend_xmin,
-            S.query
+            S.query,
+            S.backend_type
     FROM pg_stat_get_activity(NULL) AS S
         LEFT JOIN pg_database AS D ON (S.datid = D.oid)
         LEFT JOIN pg_authid AS U ON (S.usesysid = U.oid);