Add information about WAL buffers being full to EXPLAIN (WAL)

This is similar to ce5bcc4a9f, relying on the addition of
wal_buffers_full to WalUsage.  This time, the information is added to
the output generated by EXPLAIN (WAL).

Author: Bertrand Drouvot
Reviewed-by: Ilia Evdokimov
Discussion: https://postgr.es/m/Z6SOha5YFFgvpwQY@ip-10-97-1-34.eu-west-3.compute.internal

doc/src/sgml/ref/explain.sgml

@@ -242,8 +242,9 @@ ROLLBACK;
     <listitem>
      <para>
       Include information on WAL record generation. Specifically, include the
-      number of records, number of full page images (fpi) and the amount of WAL
+      number of records, number of full page images (fpi), the amount of WAL
-      generated in bytes. In text format, only non-zero values are printed.
+      generated in bytes and the number of times the WAL buffers became full.
+      In text format, only non-zero values are printed.
       This parameter may only be used when <literal>ANALYZE</literal> is also
       enabled.  It defaults to <literal>FALSE</literal>.
      </para>

src/backend/commands/explain.c

@@ -4242,7 +4242,7 @@ show_wal_usage(ExplainState *es, const WalUsage *usage)
 	{
 		/* Show only positive counter values. */
 		if ((usage->wal_records > 0) || (usage->wal_fpi > 0) ||
-			(usage->wal_bytes > 0))
+			(usage->wal_bytes > 0) || (usage->wal_buffers_full > 0))
 		{
 			ExplainIndentText(es);
 			appendStringInfoString(es->str, "WAL:");
@@ -4256,6 +4256,9 @@ show_wal_usage(ExplainState *es, const WalUsage *usage)
 			if (usage->wal_bytes > 0)
 				appendStringInfo(es->str, " bytes=" UINT64_FORMAT,
 								 usage->wal_bytes);
+			if (usage->wal_buffers_full > 0)
+				appendStringInfo(es->str, " buffers full=%lld",
+								 (long long) usage->wal_buffers_full);
 			appendStringInfoChar(es->str, '\n');
 		}
 	}
@@ -4267,6 +4270,8 @@ show_wal_usage(ExplainState *es, const WalUsage *usage)
 							   usage->wal_fpi, es);
 		ExplainPropertyUInteger("WAL Bytes", NULL,
 								usage->wal_bytes, es);
+		ExplainPropertyInteger("WAL Buffers Full", NULL,
+							   usage->wal_buffers_full, es);
 	}
 }