pgstat: split different types of stats into separate files.

pgstat.c is very long, and it's hard to find an order that makes sense and is
likely to be maintained over time. Splitting the different pieces into
separate files makes that a lot easier.

With a few exceptions, this commit just moves code around. Those exceptions
are:
- adding file headers for new files
- removing 'static' from functions
- adapting pgstat_assert_is_up() to work across TUs
- minor comment adjustments
git diff --color-moved=dimmed-zebra is very helpful separating code movement
from code changes.

The next commit in this series will reorder pgstat.[ch] contents to be a bit
more coherent.

Earlier revisions of this patch had "global" statistics (archiver, bgwriter,
checkpointer, replication slots, SLRU, WAL) in one file, because each seemed
small enough. However later commits will increase their size and their
aggregate size is not insubstantial. It also just seems easier to split each
type of statistic into its own file.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

src/backend/utils/activity/pgstat_archiver.c

@@ -0,0 +1,44 @@
+/* -------------------------------------------------------------------------
+ *
+ * pgstat_archiver.c
+ *	  Implementation of archiver statistics.
+ *
+ * This file contains the implementation of archiver statistics. It is kept
+ * separate from pgstat.c to enforce the line between the statistics access /
+ * storage implementation and the details about individual types of
+ * statistics.
+ *
+ * Copyright (c) 2001-2022, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/backend/utils/activity/pgstat_archiver.c
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "utils/pgstat_internal.h"
+#include "utils/timestamp.h"
+
+
+/* ----------
+ * pgstat_send_archiver() -
+ *
+ *	Tell the collector about the WAL file that we successfully
+ *	archived or failed to archive.
+ * ----------
+ */
+void
+pgstat_send_archiver(const char *xlog, bool failed)
+{
+	PgStat_MsgArchiver msg;
+
+	/*
+	 * Prepare and send the message
+	 */
+	pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_ARCHIVER);
+	msg.m_failed = failed;
+	strlcpy(msg.m_xlog, xlog, sizeof(msg.m_xlog));
+	msg.m_timestamp = GetCurrentTimestamp();
+	pgstat_send(&msg, sizeof(msg));
+}