1 files changed, 31 insertions, 1 deletions
diff --git a/synapse/metrics/metric.py b/synapse/metrics/metric.py
index ff5aa8c0e1..89bd47c3f7 100644
--- a/synapse/metrics/metric.py
+++ b/synapse/metrics/metric.py
@@ -115,7 +115,7 @@ class CounterMetric(BaseMetric):
         # dict[list[str]]: value for each set of label values. the keys are the
         # label values, in the same order as the labels in self.labels.
         #
-        # (if the metric is a scalar, the (single) key is the empty list).
+        # (if the metric is a scalar, the (single) key is the empty tuple).
         self.counts = {}
 
         # Scalar metrics are never empty
@@ -145,6 +145,36 @@ class CounterMetric(BaseMetric):
         )
 
 
+class GaugeMetric(BaseMetric):
+    """A metric that can go up or down
+    """
+
+    def __init__(self, *args, **kwargs):
+        super(GaugeMetric, self).__init__(*args, **kwargs)
+
+        # dict[list[str]]: value for each set of label values. the keys are the
+        # label values, in the same order as the labels in self.labels.
+        #
+        # (if the metric is a scalar, the (single) key is the empty tuple).
+        self.guages = {}
+
+    def set(self, v, *values):
+        if len(values) != self.dimension():
+            raise ValueError(
+                "Expected as many values to inc() as labels (%d)" % (self.dimension())
+            )
+
+        # TODO: should assert that the tag values are all strings
+
+        self.guages[values] = v
+
+    def render(self):
+        return flatten(
+            self._render_for_labels(k, self.guages[k])
+            for k in sorted(self.guages.keys())
+        )
+
+
 class CallbackMetric(BaseMetric):
     """A metric that returns the numeric value returned by a callback whenever
     it is rendered. Typically this is used to implement gauges that yield the