Add _routing to SQL includes list (#277)

[acarbonetto]

Description

Add the _routing reserved word as a metafield accessible as an identifier from any opensearch index.

Example:

select _id, _index, _routing, int0 from calcs limit 5

Example response:

        [
            "1",
            "calcs",
            "[xoyIAOdqQsS74x2yKSgiEQ][calcs][0]",
            1
        ],

Issues Resolved

Partially resolves: sql#1478

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[Yury-Fridlyand]

I think we need an IT to prove that filtering by _routing works fine, for example:

select *, _id from <index> where _routing = 'a'
select *, _id from <index> where _routing = 'b'

The test should propagate the data properly into different shards and get their names using REST API.

[codecov]

Codecov Report

Merging #1771 (1c0b35d) into main (fa840e0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1771   +/-   ##
=========================================
  Coverage     97.32%   97.32%           
- Complexity     4458     4459    +1     
=========================================
  Files           388      388           
  Lines         11050    11052    +2     
  Branches        790      791    +1     
=========================================
+ Hits          10754    10756    +2     
  Misses          289      289           
  Partials          7        7           

Flag	Coverage Δ
sql-engine	97.32% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
...search/sql/opensearch/storage/OpenSearchIndex.java	100.00% <ø> (ø)
...ch/sql/opensearch/response/OpenSearchResponse.java	100.00% <100.00%> (ø)

[acarbonetto]

I think we need an IT to prove that filtering by _routing works fine, for example:

select *, _id from <index> where _routing = 'a'
select *, _id from <index> where _routing = 'b'

The test should propagate the data properly into different shards and get their names using REST API.

Right now routing shard names are randomly assigned, so I cannot filter by shard name. Yet.
In the following PR, I will be able to assign documents to specific shards, and will be able to filter by routing shard using exactly what you were suggesting.

Note: this is also why a doctest with _routing doesn't work, because the documents are assigned to random shards.

[Yury-Fridlyand]

Yes, but you can get shard names with REST API or CAT API

[dai-chen]

Thanks for the changes!