common libraries for Prometheus components

 This package includes:
 .
  * github.com/prometheus/common/config: Common configuration structures.
  * github.com/prometheus/common/expfmt: Decoding and encoding for the
    exposition format.
  * github.com/prometheus/common/model: Shared data structures.
  * github.com/prometheus/common/promlog: Defines standardised ways to
    initialize Go kit loggers across Prometheus components.
  * github.com/prometheus/common/route: A routing wrapper around
    https://github.com/julienschmidt/httprouter using `context.Context`.
  * github.com/prometheus/common/server: Common servers.
  * github.com/prometheus/common/sigv4: Provides a http.RoundTripper that
    signs requests using Amazon's Signature Verification V4 signing procedure.
  * github.com/prometheus/common/version: Version information and metrics.