Instrumentation: Update to populate missing 'sample_rate_hz` attributes

To conform with the Instrumentation API each instrument should populate
the `sample_rate_hz` attribute with the relevant information.

devlib/instrument/acmecape.py

@@ -46,6 +46,7 @@ class AcmeCapeInstrument(Instrument):
         self.host = host
         self.iio_device = iio_device
         self.buffer_size = buffer_size
+        self.sample_rate_hz = 100
         if self.iio_capture is None:
             raise HostError('Missing iio-capture binary')
         self.command = None

devlib/instrument/frames.py

@@ -16,6 +16,7 @@ class FramesInstrument(Instrument):
         self.collector_target = collector_target
         self.period = period
         self.keep_raw = keep_raw
+        self.sample_rate_hz = 1 / self.period
         self.collector = None
         self.header = None
         self._need_reset = True

devlib/instrument/gem5power.py

@@ -51,6 +51,9 @@ class Gem5PowerInstrument(Instrument):
             self.add_channel(field, 'power')
         self.target.gem5stats.book_roi(self.roi_label)
         self.sample_period_ns = 10000000
+        # Sample rate must remain unset as gem5 does not provide samples
+        # at regular intervals therefore the reported timestamp should be used.
+        self.sample_rate_hz = None
         self.target.gem5stats.start_periodic_dump(0, self.sample_period_ns)
         self._base_stats_dump = 0