deps: update to Beam 2.18

Author: olavloite

dataflow/spanner-io/pom.xml

@@ -38,7 +38,7 @@
     <java.version>1.8</java.version>
     <maven.compiler.source>1.8</maven.compiler.source>
     <maven.compiler.target>1.8</maven.compiler.target>
-    <apache_beam.version>2.17.0</apache_beam.version>
+    <apache_beam.version>2.18.0</apache_beam.version>
   </properties>
 
   <build>
@@ -79,6 +79,11 @@
           <groupId>com.google.api.grpc</groupId>
           <artifactId>grpc-google-common-protos</artifactId>
         </exclusion>
+        <!-- The BigTable client brings in an OpenCensus version that conflicts with Spanner. -->
+        <exclusion>
+          <groupId>com.google.cloud.bigtable</groupId>
+          <artifactId>bigtable-client-core</artifactId>
+        </exclusion>
       </exclusions>
     </dependency>