implementations of encode_family on text & protobuf are not symmetric

[codefever]

For text (https://github.com/prometheus/client_rust/blob/master/src/encoding/text.rs#L387), family inheritance is prohibited by debug_assert.
But for protobuf (https://github.com/prometheus/client_rust/blob/master/src/encoding/protobuf.rs#L236), the iniheritantce is allowed.

Is there any special consideration? I found these when i was implementing a custom metric which would extend its own families under another prometheus_client::metrics::family::Family context.

[mxinden]

If I recall correctly, I disallowed a family in a family back then, to prevent edge cases.

What is your use-case for a family in a family?

[codefever]

If I recall correctly, I disallowed a family in a family back then, to prevent edge cases.

What is your use-case for a family in a family?

I wanted to implement a custom metric type which generate multiple metrics out with different prefix and labels (Like histogram).

After a deeper look at Summary type, I thought it quitely matches my use case. However, there's no Summary supported in this crate. I've seen several PRs here. What're the blocking issues about them...