Rename `x-goog-header-params` to `x-goog-request-params` (#5495)

diff --git a/google/api_core/gapic_v1/routing_header.py b/google/api_core/gapic_v1/routing_header.py
index a9626ee..ca35116 100644
--- a/google/api_core/gapic_v1/routing_header.py
+++ b/google/api_core/gapic_v1/routing_header.py
@@ -22,7 +22,7 @@
 
 from six.moves.urllib.parse import urlencode
 
-ROUTING_METADATA_KEY = 'x-goog-header-params'
+ROUTING_METADATA_KEY = 'x-goog-request-params'
 
 
 def to_routing_header(params):