feat(bigtable): cheap Table creation with different resource name

[dbolduc]

Related to #5934

We have customers that use our Table clients to interact with tables spread across various instances and projects. Because the project and instance ids are stored in the DataClient, the customer is forced to create multiple connections, which is redundant and slow. While this issue will be addressed as part of the effort to modernize the Data API, that will take time. This PR doesn't hurt, and it adds value now.

This PR stores the project and instance ids in the Table. Customers can now produce a copy of a Table with a modified resource name, without needing to make a new connection to the service.

// Old code
auto t1 = Table(MakeDataClient("project-1", "instance-A"), "table-X");
auto t2 = Table(MakeDataClient("project-2", "instance-B"), "table-Y"); // initiates an extra connection

// New code
auto t1 = Table(MakeDataClient("project-1", "instance-A"), "table-X");
auto t2 = t1.WithNewTarget("project-2", "instance-B", "table-Y"); // cheap

I think the PR is in line with our thread safety guarantees, but the reviewer should check this.

google-cloud-cpp/google/cloud/bigtable/table.h

Lines 86 to 90 in 9e5ac67

	* @par Thread-safety
	* Instances of this class created via copy-construction or copy-assignment
	* share the underlying pool of connections. Access to these copies via multiple
	* threads is guaranteed to work. Two threads operating concurrently on the same
	* instance of this class is not guaranteed to work.

Should there be another version of this method that sets the app profile id as well?

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[codecov]

Codecov Report

Merging #8172 (033da4a) into main (2b4f74d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #8172   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files        1323     1323           
  Lines      118191   118256   +65     
=======================================
+ Hits       111821   111884   +63     
- Misses       6370     6372    +2     

Impacted Files	Coverage Δ
...ogle/cloud/bigtable/metadata_update_policy_test.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/table.h	100.00% <100.00%> (ø)
google/cloud/bigtable/table_test.cc	100.00% <100.00%> (ø)
.../cloud/storage/benchmarks/throughput_experiment.cc	74.37% <0.00%> (-0.51%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.10% <0.00%> (-0.24%)	⬇️
google/cloud/examples/grpc_credential_types.cc	89.74% <0.00%> (+1.02%)	⬆️

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[coryan]

It strikes me as odd that I can change the project and instance ids, but not the table id? That seems like a fairly uncommon use-case?

[coryan]

An alternative would be:

Table WithNewTarget(std::string instance_id, std::string project_id, std::string table_id) const {
  auto table = *this;
  table.instance_id_ = std::move(instance_id);
  table.project_id_ = std::move(project_id);
  table.table_id_ = std::move(table_id);
  return table;
}

[dbolduc]

Yeah. Thanks for the idea.

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