instrument/daq: Add an explicit time column to the DAQ measurements

Add the monotonic clock time to the energy measurements to help
correlate the measurement with those of other collectors, like
FtraceCollector or LogcatCollector.

src/get_clock_monotonic/Makefile

@@ -0,0 +1,6 @@
+CFLAGS=-Wall --pedantic-errors -O2 -static
+
+all: get_clock_monotonic
+
+get_clock_monotonic: get_clock_monotonic.c
+	$(CC) $(CFLAGS) $^ -o $@

src/get_clock_monotonic/get_clock_monotonic.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+int main(void) {
+    int ret;
+    struct timespec tp;
+
+    ret = clock_gettime(CLOCK_MONOTONIC, &tp);
+    if (ret) {
+        perror("clock_gettime()");
+        return EXIT_FAILURE;
+    }
+
+    printf("%ld.%ld\n", tp.tv_sec, tp.tv_nsec);
+
+    return EXIT_SUCCESS;
+}