Make sure the service.environment is always set

[gregkalapos]

• When Microsoft.Extensions.Configuration.IConfiguration is passed (e.g. ASP.NET Core) we read builder.ApplicationServices.GetEnvironmentName()
• On ASP.NET classic we actually pass null
• The EnvironmentConfigurationReader (which is the default one if you don't configure the agent) also returns null.

Task:

• make sure service.environment is always set to a default, which is production
• make sure this is also covered in the docs.

[russcam]

To resolve this, should

1. FullFrameworkConfigReader pass a value of production for defaultEnvironmentName in the call to the base ctor?

or

2. AbstractConfigurationReader return production when kv inside ParseEnvironment is null (which would be inherited by EnvironmentConfigurationReader) ?