Terraform template for Reverse replication for sharded MySQL (#1946)

* Reverse replication Terraform template

* Add support for metadata db and changestream creation

* End to end working draft

* Make target_tags rule optional

* Update variables for gCloud null resource and test with VPC

* Address comments

v2/spanner-to-sourcedb/terraform/samples/README.md

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+## Terraform samples for reverse replication
+
+This repository provides samples for common scenarios users might have while trying to run a replication from Spanner to a source database.
+
+Pick a sample that is closest to your use-case, use it as a starting point, and tailor it to your own specific needs.
+
+## Other Sample Repositories
+
+The following sample repositories provide additional examples -
+
+1. [Sample environment setups](https://github.com/GoogleCloudPlatform/DataflowTemplates/blob/main/v2/spanner-common/terraform/samples)
+2. [Bulk migration](https://github.com/GoogleCloudPlatform/DataflowTemplates/blob/main/v2/sourcedb-to-spanner/terraform/samples)
+3. [Live migration](https://github.com/GoogleCloudPlatform/DataflowTemplates/blob/main/v2/datastream-to-spanner/terraform/samples/)
+
+## List of examples
+
+1. [Spanner to sharded MySQL](spanner-to-sharded-mysql/README.md)
+
+## Sample structure
+
+Each sample contains the following (and potentially more) files -
+
+1. `main.tf` - This contains the Terraform resources which will be created.
+2. `outputs.tf` - This declares the outputs that will be output as part of
+   running the terraform example.
+3. `variables.tf` - This declares the input variables that are required to
+   configure the resources.
+4. `terraform.tf` - This contains the required providers and APIs/project
+   configurations for the sample.
+5. `terraform.tfvars` - This contains the dummy inputs that need to be populated
+   to run the example.
+6. `terraform_simple.tfvars` - This contains the minimal list of dummy inputs
+   that need to be populated to run the example.
+
+## How to add a new sample
+
+It is strongly recommended to copy an existing sample and modify it according to the scenario you are trying to cover.
+This ensures uniformity in the style in which terraform samples are written.
+
+```shell
+mkdir my-new-sample
+cp -r spanner-to-sharded-mysql/* my-new-sample/
+```