docs: migrating Bigtable workload generator example to not use Hbase

[billyjacobson]

Description

Fixes #

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Checklist

• I have followed Sample Format Guide
• pom.xml parent set to latest shared-configuration
• Appropriate changes to README are included in PR
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Tests pass: mvn clean verify required
• Lint passes: mvn -P lint checkstyle:check required
• Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[igorbernstein2]

also, you might want to add a catch to rollback the refCount if you failed to instantiate the client

[billyjacobson]

If the client fails to initiate won't the whole program exit from the exception? I could add the catch and just print that the connection failed, but for the purposes of this example I feel like it is fine to just fail?