docs: All samples should include all necessary code

[danieljbruce]

Description

This PR is an extension of https://github.com/googleapis/nodejs-bigtable/pull/1720. The basic idea is that the rest of the samples should include all the code necessary to get the sample running.

Impact

It makes it easier for users to use our samples.

Testing

These are changes to the samples so they are covered by samples tests

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 12 region tags.

• samples/tableadmin.js:60, tag bigtable_list_tables
• samples/tableadmin.js:77, tag bigtable_get_table_metadata
• samples/tableadmin.js:101, tag bigtable_create_family_gc_max_age
• samples/tableadmin.js:139, tag bigtable_create_family_gc_max_versions
• samples/tableadmin.js:175, tag bigtable_create_family_gc_union
• samples/tableadmin.js:212, tag bigtable_create_family_gc_intersection
• samples/tableadmin.js:248, tag bigtable_create_family_gc_nested
• samples/tableadmin.js:291, tag bigtable_list_column_families
• samples/tableadmin.js:319, tag bigtable_update_gc_rule
• samples/tableadmin.js:352, tag bigtable_family_get_gc_rule
• samples/tableadmin.js:371, tag bigtable_delete_family
• samples/tableadmin.js:400, tag bigtable_delete_table

You are about to delete 12 region tags.

• samples/tableadmin.js:58, tag bigtable_list_tables
• samples/tableadmin.js:68, tag bigtable_get_table_metadata
• samples/tableadmin.js:84, tag bigtable_create_family_gc_max_age
• samples/tableadmin.js:114, tag bigtable_create_family_gc_max_versions
• samples/tableadmin.js:142, tag bigtable_create_family_gc_union
• samples/tableadmin.js:171, tag bigtable_create_family_gc_intersection
• samples/tableadmin.js:199, tag bigtable_create_family_gc_nested
• samples/tableadmin.js:234, tag bigtable_list_column_families
• samples/tableadmin.js:254, tag bigtable_update_gc_rule
• samples/tableadmin.js:279, tag bigtable_family_get_gc_rule
• samples/tableadmin.js:290, tag bigtable_delete_family
• samples/tableadmin.js:314, tag bigtable_delete_table

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