Fix prometheus endpoint issue (tensorflow#786)

This PR fixes prometheus endpoint issue where `%d` should be '%s'

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

tensorflow_io/core/python/ops/prometheus_dataset_ops.py

@@ -39,7 +39,7 @@ def __init__(self,
       if offset is not None:
         metadata.append("offset=%d" % offset)
       if endpoint is not None:
-        metadata.append("endpoint=%d" % endpoint)
+        metadata.append("endpoint=%s" % endpoint)
       resource, metrics = golang_ops.io_prometheus_readable_init(
           query, metadata)
       # Construct spec in eager mode, and take spec from user in graph mode.

tests/test_io_dataset_eager.py

@@ -199,7 +199,8 @@ def fixture_prometheus():
   offset = int(round(time.time() * 1000))
   args = "coredns_dns_request_count_total"
   def func(q):
-    v = tfio.experimental.IODataset.from_prometheus(q, 5, offset=offset)
+    v = tfio.experimental.IODataset.from_prometheus(
+        q, 5, offset=offset, endpoint="http://localhost:9090")
     v = v.map(lambda timestamp, value: (
         timestamp,
         value['coredns']['localhost:9153']['coredns_dns_request_count_total']))
@@ -230,7 +231,8 @@ def func(q):
                     'up': tf.TensorSpec([], tf.float64),
                 },
             },
-        })
+        },
+        endpoint="http://localhost:9090")
     v = v.map(lambda _, value: (
         value['coredns']['localhost:9153']['up'],
         value['prometheus']['localhost:9090']['up']))