Add Bigtable Generation Metadata (#1432)

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:
```
synthtool > Cloning googleapis.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b
Status: Image is up to date for googleapis/artman:latest
synthtool > Running generator for google/bigtable/artman_bigtable.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-bigtable-v2.
synthtool > No files in sources /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-bigtable-v2/proto/src/Google/Cloud/BigTable were copied. Does the source contain files?
synthtool > Running generator for google/bigtable/admin/artman_bigtableadmin.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-bigtable-admin-v2.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/BigtableGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/BigtableClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/V2/BigtableTableAdminClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/V2/BigtableInstanceAdminClientTest.php.
synthtool > Replaced 'Cluster_State' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'Instance_Type' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'Instance_State' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'CreateTableRequest_Split' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'ModifyColumnFamiliesRequest_Modification' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'Table_View' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'MutateRowsRequest_Entry' in src/V2/Gapic/BigtableGapicClient.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

```

Bigtable/synth.metadata

@@ -0,0 +1,27 @@
+{
+  "sources": [
+    {
+      "git": {
+        "name": "googleapis",
+        "remote": "https://github.com/googleapis/googleapis.git",
+        "sha": "5a57f0c13a358b2b15452bf2d67453774a5f6d4f",
+        "internalRef": "221837528"
+      }
+    },
+    {
+      "git": {
+        "name": "googleapis-private",
+        "remote": "https://github.com/googleapis/googleapis-private.git",
+        "sha": "6aa8e1a447bb8d0367150356a28cb4d3f2332641",
+        "internalRef": "221340946"
+      }
+    },
+    {
+      "generator": {
+        "name": "artman",
+        "version": "0.16.0",
+        "dockerImage": "googleapis/artman@sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b"
+      }
+    }
+  ]
+}