fix: preserve hooks for added writer interfaces (felixge#35)

* fix: preserve hooks for added writer interfaces

* fix: preserve FlushError errors through Flush hooks

codegen: fix generated header comment (felixge#25)

Allow specifying initial metrics.

Remove mutex and add test for ReaderFrom

Resolves felixge#13

add go.mod file

Version 1

This release is used in many production deployments and there are no
known problems.