pmstat: add -C/--compact mode to hide unavailable metrics

This change adds optional dynamic column hiding to pmstat, allowing it
to automatically suppress columns for metrics that are unavailable on
the current platform.

This is particularly useful for macOS and other platforms where some
Linux-specific metrics (like swap in/out, buffer memory, etc.) don't
exist. Instead of showing "?" for missing values, the columns are
simply not displayed.

Usage:
  pmstat          # Default: show all columns (original behavior)
  pmstat -C       # Compact: hide unavailable metric columns

Note: This commit is part of separating cross-platform tool improvements
from platform-specific PMDA enhancements. Darwin PMDA swap metrics have
been split into a separate PR for the macOS-specific enhancement branch.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: tallpsmith

src/pmstat/pmstat.c

@@ -113,6 +113,7 @@ pmLongOptions longopts[] = {
     PMOPT_HOSTSFILE,
     PMOPT_LOCALPMDA,
     PMAPI_OPTIONS_HEADER("Reporting options"),
+    { "compact", 0, 'C', 0, "suppress columns for unavailable metrics" },
     { "suffix", 0, 'l', 0, "print last 7 charcters of the host name(s)" },
     { "pause", 0, 'P', 0, "pause between updates for archive replay" },
     { "xcpu", 0, 'x', 0, "extended CPU statistics reporting" },
@@ -121,11 +122,12 @@ pmLongOptions longopts[] = {
 
 pmOptions opts = {
     .flags = PM_OPTFLAG_MULTI | PM_OPTFLAG_BOUNDARIES | PM_OPTFLAG_STDOUT_TZ,
-    .short_options = PMAPI_OPTIONS "H:LlPx",
+    .short_options = PMAPI_OPTIONS "CH:LlPx",
     .long_options = longopts,
 };
 
 static int extraCpuStats;
+static int compactMode;
 static char swapOp = 'p';
 static int rows = 21;
 static int header;
@@ -436,7 +438,25 @@ detectColumnAvailability(struct statsrc **ctxList, int ctxCount)
     /* Start optimistic - assume all columns available */
     memset(&display, 1, sizeof(display));
 
-    /* Check each context */
+    /*
+     * If not in compact mode, show all columns regardless of availability.
+     * Unavailable metrics will be displayed as "?" in the output.
+     */
+    if (!compactMode) {
+	/* Set all group visibility to show all groups */
+	for (i = 0; i < NUM_COL_GROUPS; i++)
+	    display.show_group[i] = 1;
+
+	/* Verbose reporting - only if debugging enabled */
+	if (pmDebugOptions.appl0)
+	    reportAvailabilityDiagnostics();
+	return;
+    }
+
+    /*
+     * Compact mode: analyze availability and suppress unavailable columns.
+     * Check each context and mark columns unavailable if ANY context lacks them.
+     */
     for (i = 0; i < ctxCount; i++) {
 	struct statsrc *s = ctxList[i];
 
@@ -766,6 +786,9 @@ main(int argc, char *argv[])
 
     while ((sts = pmGetOptions(argc, argv, &opts)) != EOF) {
 	switch (sts) {
+	case 'C':	/* compact mode - suppress unavailable columns */
+	    compactMode = 1;
+	    break;
 	case 'l':	/* print last 7 characters of hostname(s) */
 	    printTail++;
 	    break;