Hide Config#create() method and use builder everywhere

[mateuszrzeszutek]

Config#create() does not normalize property names, which could cause some confusion when writing tests. Now it was made private and ConfigBuilder is used everywhere to create Config instances.

Also added a Config#getLongProperty() that might be useful for vendor distros.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Mateusz Rzeszutek (b0ad1b6)

[trask]

Can we remove/deprecate Config.asJavaProperties() now that getAllProperties() is public?

[mateuszrzeszutek]

Can we remove/deprecate Config.asJavaProperties() now that getAllProperties() is public?

Unfortunately I can't remove it (it's still used in e.g. SpanExporterFactory, which is deprecated by the way), but it definitely should be deprecated 👍