Split backend status and progress related functionality out of pgstat.c.

Backend status (supporting pg_stat_activity) and command
progress (supporting pg_stat_progress*) related code is largely
independent from the rest of pgstat.[ch] (supporting views like
pg_stat_all_tables that accumulate data over time). See also
a333476b92.

This commit doesn't rename the function names to make the distinction
from the rest of pgstat_ clearer - that'd be more invasive and not
clearly beneficial. If we were to decide to do such a rename at some
point, it's better done separately from moving the code as well.

Robert's review was of an earlier version.

Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/20210316195440.twxmlov24rr2nxrg@alap3.anarazel.de

src/include/utils/backend_progress.h

@@ -0,0 +1,44 @@
+/* ----------
+ * backend_progress.h
+ *	  Command progress reporting definition.
+ *
+ * Note that this file provides the infrastructure for storing a single
+ * backend's command progress counters, without ascribing meaning to the
+ * individual fields. See commands/progress.h and system_views.sql for that.
+ *
+ * Copyright (c) 2001-2021, PostgreSQL Global Development Group
+ *
+ * src/include/utils/backend_progress.h
+ * ----------
+ */
+#ifndef BACKEND_PROGRESS_H
+#define BACKEND_PROGRESS_H
+
+
+/* ----------
+ * Command type for progress reporting purposes
+ * ----------
+ */
+typedef enum ProgressCommandType
+{
+	PROGRESS_COMMAND_INVALID,
+	PROGRESS_COMMAND_VACUUM,
+	PROGRESS_COMMAND_ANALYZE,
+	PROGRESS_COMMAND_CLUSTER,
+	PROGRESS_COMMAND_CREATE_INDEX,
+	PROGRESS_COMMAND_BASEBACKUP,
+	PROGRESS_COMMAND_COPY
+} ProgressCommandType;
+
+#define PGSTAT_NUM_PROGRESS_PARAM	20
+
+
+extern void pgstat_progress_start_command(ProgressCommandType cmdtype,
+										  Oid relid);
+extern void pgstat_progress_update_param(int index, int64 val);
+extern void pgstat_progress_update_multi_param(int nparam, const int *index,
+											   const int64 *val);
+extern void pgstat_progress_end_command(void);
+
+
+#endif /* BACKEND_PROGRESS_H */