add log warning to starlark drop-fields example

(cherry picked from commit 42eacb3)

plugins/processors/starlark/testdata/drop_fields_with_unexpected_type.star

@@ -10,17 +10,21 @@
 # measurement,host=hostname a=1i,b=4.2,c=42.0,d="v3.14",e=true,f=23.0 1597255410000000000
 # measurement,host=hostname a=1i,c=42.0,d="v3.14",e=true,f=23.0 1597255410000000000
 
+load("logging.star", "log")
+# loads log.debug(), log.info(), log.warn(), log.error()
+
 expected_type = {
     "a": "int",
     "b": "float",
     "c": "float",
     "d": "string",
     "e": "bool"
 }
-    
+
 def apply(metric):
     for k, v in metric.fields.items():
         if type(v) != expected_type.get(k, type(v)):
             metric.fields.pop(k)
+            log.warn("Unexpected field type dropped: metric {} had field {} with type {}, but it is expected to be {}".format(metric.name, k, type(v), expected_type.get(k, type(v))))
 
     return metric