Make DbContext pooling accessible to non-DI applications

[roji]

The setup of DbContext pooling is currently tied to DI. While it's possible to set it up without DI, that currently requires using internal types.

[zliebersbach]

@roji I notice you mention it is possible to set up pooling without DI, would you be able to cite any examples of how to implement this? It would be an amazing help!

[roji]

Here's a way to do it:

var optionsBuilder = new DbContextOptionsBuilder<FortunesContext>();
// Configure context options
var pool = new DbContextPool<FortunesContext>(optionsBuilder.options);

// Now that you have a pool, you can use it as follows:

var poolable = _contextPool.Rent();
poolable.SetLease(new DbContextLease(_contextPool, standalone: true));
using var dbContext = (FortunesContext)poolable;

However, note again the above uses internal APIs, and so may break in future releases.

[roji]

As discussed with @ajcvickers offline, bypassing DI in the TE Fortunes benchmark yields around 2.8% RPS. So I'll be doing this for 6.0.0.