Add "_bucket" suffix to histogram Samples

[DeusExLibris]

Issue addressed by this PR:
According to the upstream documentation here - bucket counts should take the form _bucket{le="..."}. However, the current code does not add the "_bucket" suffix.

Background:
I discovered this trying to have DataDog import the bucket values. OpenMetrics Histograms were not being processed correctly by DataDog after verifying that I had setup the agent correctly. DataDog was capturing the _count and _sum values but not the buckets. Once I made the change in this PR, the agent started loading the histograms properly.

[fredrikekre]

Thanks! I was sure this wasn't in the docs but https://prometheus.io/docs/concepts/metric_types/#histogram mentions this so I don't know why I missed it. I would also note that Grafana seems happy without the _bucket suffix which is why I didn't notice it.

Do you want to update the tests? I think you only have to update

Prometheus.jl/test/runtests.jl

Line 223 in fc768fb

@test sample.suffix === nothing

,

Prometheus.jl/test/runtests.jl

Lines 236 to 238 in fc768fb

	metric_name_histogram{le="1.0"} 1
	metric_name_histogram{le="2.0"} 2
	metric_name_histogram{le="Inf"} 2

and

Prometheus.jl/test/runtests.jl

Line 494 in fc768fb

@test sample.suffix === nothing

[DeusExLibris]

Yes - Grafana does seem to only care that the metric is set to histogram and handles the buckets automatically. This is, in fact, how I discovered the issue. My code was working fine in Grafana but I couldn't get it to work in DataDog.

I will update the tests and submit a new PR.