Use an interface instead of a concrete type

[pavelnikolov]

I am using the following versions:

- package: github.com/prometheus/client_golang
  version: ~0.9.0-pre1
- package: github.com/uber/jaeger-client-go
  version: ^2.12.0
# only in my glide.lock file
- name: github.com/uber/jaeger-lib
  version: 4267858c0679cd4e47cefed8d7f70fd386cfb567

If we use a histogram and try to create a new tracer (using jaeger-client-go) with metrics, for example:

        var reg prometheus.Registerer
        // set the value of reg to some Prometheus registry here
	factory := jaegerprom.New(jaegerprom.WithRegisterer(reg))
	tracer, closer, err := config.New(appName, jaegercfg.Metrics(factory))

I get the following error:

vendor/github.com/uber/jaeger-lib/metrics/prometheus/factory.go:143:12: cannot use hv.WithLabelValues(f.tagsAsLabelValues(labelNames, tags)...) (type prometheus.Observer) as type prometheus.Histogram in field value:
	prometheus.Observer does not implement prometheus.Histogram (missing Collect method)

This PR fixes this issue, while all unit tests still pass. It just replaces a concrete type with a single-method interface.

[pavelnikolov]

I noticed that you use version 0.8.0 of the prometheus client for Go... This is the reason why the build fails : (
There are two options to fix this:

1. update the prometheus client version referenced by this project
2. create a copy of the prometheus.Observer single-method interface

Please, advise.

[pavelnikolov]

I just read the comments on #33 it seems that this PR can solve this problem.
If you don't want to update the version of Prometheus, then I'd suggest creating an Observer interface with a single method Observe(v float64). Then use it instead of a histogram, for example:

type observer interface {
	Observe(v float64)
}

type timer struct {
	histogram observer
}

This would solve #33, without having to upgrade your dependencies to newer version of Prometheus. Once prometheus 0.9.0 is out you can delete this interface and replace it with prometheus.Observer. WDYT?

[codecov]

Codecov Report

Merging #42 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #42   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          25     25           
  Lines         710    710           
=====================================
  Hits          710    710

Impacted Files	Coverage Δ
metrics/prometheus/factory.go	100% <ø> (ø)	⬆️

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[pavelnikolov]

/assign @yurishkuro

[yurishkuro]

nice solution!

[yurishkuro]

Thanks, @pavelnikolov

[yurishkuro]

@pavelnikolov a couple questions:

• does it resolve Could not compile with newest github.com/prometheus/client_golang/prometheus #33?
• do you need a new release of jaeger-lib?

[pavelnikolov]

@yurishkuro

• Yes, it does solve Could not compile with newest github.com/prometheus/client_golang/prometheus #33.
• It'd be nice to have, but not a high priority. In the mean time I can pin the version to the git commit.

[yurishkuro]

@pavelnikolov released as 1.5.0