MongoDB - Fix Unhealthy if no collections exists in the specified database

[ThumNet]

What this PR does / why we need it:
We are experiencing unhealthy checks if a specified mongo database has no collections yet.

System.InvalidOperationException: Sequence contains no elements at System.Linq.ThrowHelper.ThrowNoElementsException() at System.Linq.Enumerable.First[TSource](IEnumerable`1 source) at MongoDB.Driver.IAsyncCursorExtensions.FirstAsync[TDocument](IAsyncCursor`1 cursor, CancellationToken cancellationToken) at HealthChecks.MongoDb.MongoDbHealthCheck.CheckHealthAsync(HealthCheckContext context, CancellationToken cancellationToken) in /_/src/HealthChecks.MongoDb/MongoDbHealthCheck.cs:line 43

Which issue(s) this PR fixes:
#617

Special notes for your reviewer:
None

Does this PR introduce a user-facing change?:
No

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[sungam3r]

Thanks.

[sungam3r]

@ThumNet One test failed. I think mongoClient.GetDatabase(_specifiedDatabase) returns an instance even for non existing database so First() method correctly failed HC before. I'm fine using FirstOrDefault but of course some other changes are required to fix failed test.

[ThumNet]

Looking into the failing test, will update the PR when fixed.

[ThumNet]

After looking into the failing test, which is called be_unhealthy_on_connectionstring_specified_database_if_mongodb_is_available_and_database_exist.

I think that this test should NOT exists, because of the way mongodb works. MongoDB only creates the database when you first store data in that database. This data could be a collection or a document.

Sources: official mongo docs and blog post

@sungam3r Do you agree the test can be removed?

[sungam3r]

I've read provided links. Do you think the current method of checking the state of MongoDB is sufficient? I have no experience with MongoDB.

[ThumNet]

Instead of getting the collections i think it is better to implement a ping to the database. Others also recommend this.

I will change the code to allow this and update the PR. Also change the test accordingly.

[ThumNet]

@sungam3r Update has just been pushed.

[sungam3r]

@ThumNet Thanks.