fix(prometheus): improve logging when having the inconsistent labels …

…count (#13020)

Currently, the Prometheus plugin will log the following error if we have encountered an inconsistent label count while debugging:

```
[error]... inconsistent labels count, expected 6, got 5
```

It's hard to identify which metric is going wrong, and it will be helpful if we can bring the metric name as well:

```
[error]... metric 'bandwidth_bytes' has the inconsistent labels count, expected 6, got 5
```

Co-authored-by: Qi <44437200+ADD-SP@users.noreply.github.com>

changelog/unreleased/kong/improve-prometheus-error-logging.yml

@@ -0,0 +1,4 @@
+message: |
+  **Prometheus**: Improved error logging when having inconsistent labels count.
+type: bugfix
+scope: Plugin

kong/plugins/prometheus/prometheus.lua

@@ -377,8 +377,8 @@ local function lookup_or_create(self, label_values)
   local cnt = label_values and #label_values or 0
   -- specially, if first element is nil, # will treat it as "non-empty"
   if cnt ~= self.label_count or (self.label_count > 0 and label_values[1] == nil) then
-    return nil, string.format("inconsistent labels count, expected %d, got %d",
-                              self.label_count, cnt)
+    return nil, string.format("metric '%s' has inconsistent labels count, expected %d, got %d",
+      self.name, self.label_count, cnt)
   end
   local t = self.lookup
   if label_values then