Integrate Python GAPIC Microgenerator in googleapis.

This PR uses using documentai as an example.
Depends on googleapis/gapic-generator-python#402

PiperOrigin-RevId: 309824146

Source-Author: Google APIs <noreply@google.com>
Source-Date: Mon May 4 15:06:44 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: e0f9d9e1f9de890db765be46f45ca8490723e3eb
Source-Link: googleapis/googleapis@e0f9d9e

Author: yoshi-automation

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>5.1.0</version>
+      <version>5.3.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>1.52.0</version>
+  <version>1.53.0</version>
 </dependency>
 
 ```

synth.metadata

@@ -4,22 +4,22 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "5806b0e1c7c5bfe7891283ae515b114b3ef814d9"
+        "sha": "2d19c25ba32847d116194565e67e1b1276fcb9f8"
       }
     },
     {
       "git": {
         "name": "googleapis",
         "remote": "https://github.com/googleapis/googleapis.git",
-        "sha": "ab5d07c2883026eec395d73ff59a17f4ad0153db",
-        "internalRef": "306557455"
+        "sha": "e0f9d9e1f9de890db765be46f45ca8490723e3eb",
+        "internalRef": "309824146"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "716f741f2d307b48cbe8a5bc3bc883571212344a"
+        "sha": "04cb397eb7590ea1e6c10a39fca3d8fe0fb3d256"
       }
     }
   ],