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_checkpointer.c

@@ -0,0 +1,61 @@
+/* -------------------------------------------------------------------------
+ *
+ * pgstat_checkpointer.c
+ *	  Implementation of checkpoint statistics.
+ *
+ * This file contains the implementation of checkpoint 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_checkpointer.c
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "utils/pgstat_internal.h"
+
+
+/*
+ * Checkpointer global statistics counters.  Stored directly in a stats
+ * message structure so they can be sent without needing to copy things
+ * around.  We assume this init to zeroes.
+ */
+PgStat_MsgCheckpointer PendingCheckpointerStats;
+
+
+/* ----------
+ * pgstat_send_checkpointer() -
+ *
+ *		Send checkpointer statistics to the collector
+ * ----------
+ */
+void
+pgstat_send_checkpointer(void)
+{
+	/* We assume this initializes to zeroes */
+	static const PgStat_MsgCheckpointer all_zeroes;
+
+	/*
+	 * This function can be called even if nothing at all has happened. In
+	 * this case, avoid sending a completely empty message to the stats
+	 * collector.
+	 */
+	if (memcmp(&PendingCheckpointerStats, &all_zeroes, sizeof(PgStat_MsgCheckpointer)) == 0)
+		return;
+
+	/*
+	 * Prepare and send the message
+	 */
+	pgstat_setheader(&PendingCheckpointerStats.m_hdr, PGSTAT_MTYPE_CHECKPOINTER);
+	pgstat_send(&PendingCheckpointerStats, sizeof(PendingCheckpointerStats));
+
+	/*
+	 * Clear out the statistics buffer, so it can be re-used.
+	 */
+	MemSet(&PendingCheckpointerStats, 0, sizeof(PendingCheckpointerStats));
+}