Remove backticks on by field in stats

[joshuali925]

Description

Current behavior:

❯ ppl 'source=accounts | stats count() by `gender`' | jq '.schema[]'
{
  "name": "count()",
  "type": "integer"
}
{
  "name": "`gender`",
  "type": "string"
}

This PR will change it to

❯ ppl 'source=accounts | stats count() by `gender`' | jq '.schema[]'
{
  "name": "count()",
  "type": "integer"
}
{
  "name": "gender",
  "type": "string"
}

Issues Resolved

[List any issues this PR will resolve]

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

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[joshuali925]

@vamsi-amazon is it expected to have output fields in backticks for

stats xxx by `field`

i thought it's a bug but seems tests is expecting fields with backticks?

sql/integ-test/src/test/java/org/opensearch/sql/ppl/PrometheusDataSourceCommandsIT.java

Lines 110 to 117 in 4e9ca01

	public void testMetricAvgAggregationCommand() {
	JSONObject response =
	executeQuery("source=`my_prometheus`.`prometheus_http_requests_total` | stats avg(@value) as `agg` by span(@timestamp, 15s), `handler`, `job`");
	verifySchema(response,
	schema("agg", "double"),
	schema("span(@timestamp,15s)", "timestamp"),
	schema("`handler`", "string"),
	schema("`job`", "string"));

[codecov]

Codecov Report

Merging #1728 (2225428) into main (94d5479) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1728   +/-   ##
=========================================
  Coverage     97.33%   97.33%           
  Complexity     4408     4408           
=========================================
  Files           388      388           
  Lines         10938    10939    +1     
  Branches        773      773           
=========================================
+ Hits          10647    10648    +1     
  Misses          284      284           
  Partials          7        7           

Flag	Coverage Δ
sql-engine	97.33% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...java/org/opensearch/sql/ppl/parser/AstBuilder.java	100.00% <100.00%> (ø)

[joshuali925]

confirmed with vamsi the tests can be changed. BWC is failing, error seems irrelevant to this PR

Execution failed for task ':integ-test:sqlBwcCluster#mixedClusterTask'.
> `cluster{:integ-test:sqlBwcCluster0}` failed to wait for cluster health yellow after 40 SECONDS
    IO error while waiting cluster
    503 Service Unavailable

[vamsi-amazon]

If the query is
"source = t | stats count(a) by `t.b` "

it would treat t.b as column name.