Remove dependency on psr/http-message

[m-nic]

I don't think this package should rely on psr/http-message.
It should be the client responsibility to expose the metrics.
At most some recommendations/examples can be provided.

Most likely many of the clients already use a framework that uses the psr7, meaning they should just hook it up themselves - also this can bring in conflicts for frameworks using psr/http-message: ^2.0 dependency.

Or as in my case, I have to bring in an unwanted dependency because I just want to dump the metrics directly to a file.

What do you think?

[hollodotme]

I'm always in for reduction of dependencies. I'll have a look, if we can leave it out.

[hollodotme]

@m-nic I agree with you and removed the classes HttpResponse and OutputStream which implemented psr/http-message interfaces. Thus removed the composer dependency as well.

Will be released in #13