`PropertyNamingPolicy` from `JsonSerializerOptions` of the `DefaultSourceSerializer` is not applied for mappings

[stevejgordon]

Elastic.Clients.Elasticsearch version: Elastic.Clients.Elasticsearch.Serverless 1.1.10, but will also affect Elastic.Clients.Elasticsearch

Elasticsearch version: Serverless

.NET runtime version: 9.0.0-rc.1

Operating system version: Win 10

Description of the problem including expected versus actual behavior:
When customising the DefaultSourceSerializer with JsonSerializerOptions that includes the PropertyNamingPolicy, that option is not honoured when manually creating an index with property mappings.

Steps to reproduce:
Create an ElasticsearchClient:

var options = sp.GetRequiredService<IOptions<ElasticsearchOptions>>().Value;

static void ConfigureOptions(JsonSerializerOptions o) =>
    o.PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower;

var nodePool = new CloudNodePool(options.CloudId, new ApiKey(options.ApiKey));
var settings = new ElasticsearchClientSettings(
    nodePool,
    sourceSerializer: (defaultSerializer, settings) =>
        new DefaultSourceSerializer(settings, ConfigureOptions));

var es = ElasticsearchClient(settings);

Create an index with mappings:

var indexResponse = await es.Indices.CreateAsync<User>(i => i
    .Index("users.v1")
    .Mappings(m => m.Properties(p => p
        .Keyword(f => f.Id)
        .Keyword(f => f.Email)
        .Date(f => f.CreatedUtc))), ct);

Actual Mapping:

{
  "users.v1": {
    "mappings": {
      "properties": {
        "createdUtc": {
          "type": "date"
        },
        "email": {
          "type": "keyword"
        },
        "id": {
          "type": "keyword"
        }
      }
    }
  }
}

Expected Mapping:

{
  "users.v1": {
    "mappings": {
      "properties": {
        "created_utc": {
          "type": "date"
        },
        "email": {
          "type": "keyword"
        },
        "id": {
          "type": "keyword"
        }
      }
    }
  }
}

NOTE: The casing of createdUtc which should be created_utc.

Expected behavior
The inferred field names in the mapping should reflect the configured PropertyNamingPolicy from System.Text.Json.

The workaround is to use the JsonPropertyName attribute on the POCO properties to manually provide a snake-cased name.

[stevejgordon]

A cleaner workaround is to manually set the DefaultFieldNameInferrer:

var settings = new ElasticsearchClientSettings(nodePool,
    sourceSerializer: (defaultSerializer, settings) =>
        new DefaultSourceSerializer(settings, ConfigureOptions))
            .DefaultFieldNameInferrer(name => JsonNamingPolicy.SnakeCaseLower.ConvertName(name));