Ensure that metric hierarchies of more than 2 levels are supported

[davidor]

Up until now, in Porta it was possible to define hierarchies of metrics with only 2 levels. This is going to change to support the "APIs as a product" feature.

For apisonator, we need to ensure that hierarchies of more than 2 levels are supported. This means that we need to check things like:

• The methods to create metrics work correctly.
• The hierarchy extension works correctly.
• The limits extension works correctly.
• Limits are applied checking all the levels of the hierarchy.
• Reports take into account all the levels of the hierarchy.
• The XML returns the info of all the relevant metrics.

Some of those things might already be working fine, but I think in the past, when implementing some features, we've assumed that it was fine to assume a max of 2 levels.

[davidor]

Done in #119 , #121 and #122

[davidor]

/cc @hallelujah @guicassolato This is one of the requirements for APIs as a product.