Add MongoClientSettingsBuilderCustomizer hook for the Mongo client

[codeconsole]

What

Adds a MongoClientSettingsBuilderCustomizer extension point to GORM for MongoDB. Any beans of this type in the application context are applied, in order, to the MongoClientSettings.Builder of the default connection source before the MongoClient is created.

Why

There is currently no supported way to customize MongoClientSettings for things that have no grails.mongodb.* configuration equivalent — most notably registering a driver com.mongodb.event.CommandListener for metrics/tracing (e.g. Micrometer's MongoMetricsCommandListener), but also programmatic connection-pool tuning, read/write concerns, etc. Today that requires replacing the datastore wiring.

Mirrors Spring Boot's MongoClientSettingsBuilderCustomizer of the same name, giving GORM-MongoDB users the same ergonomics.

How

• New functional interface org.grails.datastore.mapping.mongo.connections.MongoClientSettingsBuilderCustomizer.
• MongoConnectionSourceFactory gains an @Autowired(required = false) List<MongoClientSettingsBuilderCustomizer> applied in create() just before MongoClients.create(...). The factory is already registered with bean.autowire = true in MongoDbDataStoreSpringInitializer, so customizer beans are injected automatically. No behavior change when none are defined.

Example

@Bean
MongoClientSettingsBuilderCustomizer mongoMetrics(MeterRegistry registry) {
    return { builder -> builder.addCommandListener(new MongoMetricsCommandListener(registry)) }
}

Tests

Added to MongoConnectionSourceFactorySpec:

• a registered customizer is applied, and its command listener lands in the built settings
• multiple customizers run in registration order

[matrei]

@codeconsole Can this target 8?
Decided in the weekly meeting May 27th, point 1, 7.x.x is not accepting new features.
Focus should now be on 8+.

[codeconsole]

@codeconsole Can this target 8? Decided in the weekly meeting May 27th, point 1, 7.x.x is not accepting new features. Focus should now be on 8+.

@matrei done, how about a review ;P

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.3694%. Comparing base (098660a) to head (d37eba7).
⚠️ Report is 2 commits behind head on 8.0.x.

Files with missing lines	Patch %	Lines
...go/connections/MongoConnectionSourceFactory.groovy	50.0000%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #15716        +/-   ##
==================================================
- Coverage     48.3729%   48.3694%   -0.0035%     
- Complexity      15108      15109         +1     
==================================================
  Files            1870       1870                
  Lines           85457      85459         +2     
  Branches        14900      14901         +1     
==================================================
- Hits            41338      41336         -2     
- Misses          37784      37785         +1     
- Partials         6335       6338         +3     

Files with missing lines	Coverage Δ
...go/connections/MongoConnectionSourceFactory.groovy	60.7143% <50.0000%> (-0.8242%)	⬇️

... and 2 files with indirect coverage changes

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