Fix issue with invalid breadcrumb metadata

[tombruijn]

We had a report of breadcrumbs not working when the metadata was invalid. This required some manual intervention from us to fix the data. Avoid this occurring in the future by not allowing any other metadata than a Hash object.

Closes https://github.com/appsignal/support/issues/269

[unflxw]

Looks good as-is, some thoughts:

Would it be useful/convenient for users if add_breadcrumb tried to call .to_h on the value? (and then verified that that is a Hash)

Also, going by the docs, the keys and values should both be strings. We could check for that as well.

[tombruijn]

Would it be useful/convenient for users if add_breadcrumb tried to call .to_h on the value? (and then verified that that is a Hash)

I don't think that's necessary. We can require people to call to_h themselves. We don't do this anywhere else.

Also, going by the docs, the keys and values should both be strings. We could check for that as well.

Strings, Symbols, integers, any flat object seems to be supported. I'll add some extra checks.

[tombruijn]

Looks like the internal structure doesn't really matter, as long as the metadata object itself is a Hash. I'll leave the PR as-is.