Allow custom registration of Redis ElasticApmProfiler

[glucaci]

In order to register the profiler for Microsoft.Extensions.Caching.StackExchangeRedis we need public access to ElasticApmProfiler because the IConnectionMultiplexer is encapsulated in the RedisCache implementation.

Bellow is an example for which I have also opened a PR in dotnet/aspnetcore#31018 to allow hooking up the ProfilerSession

public static class ServiceCollectionExtensions
{
    public static void RegisterElasticApmRedis(this IServiceCollection services)
    {
        services
            .AddSingleton<IDistributedCache>(sp =>
            {
                var profiler = new ElasticApmProfiler(sp.GetService<IApmAgent>);
                var options = new RedisCacheOptions()
                {
                    Configuration = "localhost",
                    ProfilingSession = profiler.GetProfilingSession
                };

                return new RedisCache(options);
            });
    }
}

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💚 CLA has been signed

[apmmachine]

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• Build Cause: Started by user Russ Cam

• Start Time: 2021-03-22T00:19:29.577+0000

• Duration: 59 min 14 sec

• Commit: b40a7e6

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Failed	0
Passed	18759
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Total	18839

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[glucaci]

I've signed the CLA, must be re-checked

[russcam]

LGTM, thanks @glucaci 👍