Fix table formatter

It's possible for Prometheus to return multiple metrics, where
each of them has different labels. Before this the client
would fail with error when using the default table formatter,
because of for example:

Row has incorrect number of values, (actual) 8!=9 (expected)

It was also possible to mismatch the label keys and values
for some of the rows.

This patch makes sure the label values are matched to the correct
keys. It also makes sure it uses all labels and it fills
missing values with empty strings for metrics, which don't have
that label defined.

Example table output:
+------------+----------+------------+----------------+-------+
| group      | __name__ | job        | instance       | value |
+------------+----------+------------+----------------+-------+
|            | up       | sg-core    | localhost:3000 | 1     |
| production | up       | prometheus | localhost:9090 | 1     |
+------------+----------+------------+----------------+-------+

Example json output:
[
  {
    "__name__": "up",
    "group": "",
    "job": "sg-core",
    "instance": "localhost:3000",
    "value": "1"
  },
  {
    "__name__": "up",
    "group": "production",
    "job": "prometheus",
    "instance": "localhost:9090",
    "value": "1"
  }
]

Change-Id: Id0dfabf52fe0a21194c498f4aefa1bff1d3eeea9

observabilityclient/utils/metric_utils.py

@@ -98,18 +98,14 @@ def format_labels(d: dict) -> str:
 
 
 def metrics2cols(m):
-    cols = []
+    # get all label keys
+    cols = list(set().union(*(d.labels.keys() for d in m)))
+    cols.append("value")
     fields = []
-    first = True
     for metric in m:
-        row = []
+        row = [""] * len(cols)
         for key, value in metric.labels.items():
-            if first:
-                cols.append(key)
-            row.append(value)
-        if first:
-            cols.append("value")
-        row.append(metric.value)
+            row[cols.index(key)] = value
+        row[cols.index("value")] = metric.value
         fields.append(row)
-        first = False
     return cols, fields