Fetch variable metadata from Spanner

[n-h-diaz]

This PR adds support for fetching variable metadata by variable from Spanner

It reads from the new VariableMetadata table which has aggregated data per variable x facet computed via BQ federation. (Currently it's only loaded with data from CensusACS5YearSurvey import)

Note: this table is based on existing requirements to support (v1) BulkVariableInfo. If additional metadata is needed, we can modify the BQ query + Spanner schema

[gemini-code-assist]

Summary of Changes

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This pull request introduces the capability to fetch variable metadata from Spanner, utilizing a new VariableMetadata table. This table aggregates data per variable and facet, initially populated from the CensusACS5YearSurvey import, and is designed to support existing requirements for bulk variable information.

Highlights

• Spanner Client Interface Extension: The SpannerClient interface has been extended with a new method, GetVariableMetadata, enabling the retrieval of variable-specific metadata from the Spanner database.
• Variable Metadata Model: A new Go struct, VariableMetadata, was introduced to accurately model the schema of the VariableMetadata table in Spanner, facilitating structured data handling.
• Query Implementation: The GetVariableMetadata method has been implemented within the spannerDatabaseClient, including the necessary logic to query the Spanner database and map the returned data to the VariableMetadata struct.
• SQL Query Builder: A new function, GetVariableMetadataQuery, was added to programmatically construct the appropriate SQL statement for fetching variable metadata based on a list of provided variable IDs.
• Golden File Testing: Comprehensive golden file tests were introduced for both the SQL query generation and the data retrieval process, ensuring the correctness and consistency of the new functionality.

Changelog

• internal/server/spanner/client.go
  • Added GetVariableMetadata method to the SpannerClient interface.
• internal/server/spanner/golden/query/get_variable_metadata.json
  • Added a golden JSON file containing example variable metadata for testing.
• internal/server/spanner/golden/query_builder/get_variable_metadata.sql
  • Added a golden SQL file for the GetVariableMetadata query builder.
• internal/server/spanner/golden/query_builder_test.go
  • Added a new test function TestGetVariableMetadataQuery to validate the SQL query builder.
• internal/server/spanner/golden/query_cases_test.go
  • Added variableMetadataTestCases to define test inputs for variable metadata queries.
• internal/server/spanner/golden/query_test.go
  • Added a new test function TestGetVariableMetadata to validate the Spanner client's metadata retrieval.
• internal/server/spanner/model.go
  • Defined the VariableMetadata struct with fields corresponding to the Spanner table.
• internal/server/spanner/query.go
  • Implemented the GetVariableMetadata method to query Spanner and process results.
• internal/server/spanner/query_builder.go
  • Added the GetVariableMetadataQuery function to generate Spanner SQL statements.
• internal/server/spanner/statements.go
  • Declared and initialized the getVariableMetadata SQL template string.

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[gemini-code-assist]

Code Review

This pull request introduces functionality to fetch variable metadata from Spanner, complete with a new data model, query logic, and comprehensive tests using golden files. The implementation is clean and follows existing patterns. I've provided a couple of suggestions to enhance robustness and consistency: one regarding the use of SELECT * in SQL queries, and another to align error handling with other functions in the module.