examples: Refresh client_golang examples

[bwplotka]

@lilic's PR inspired me to look at our examples, and we might improve those a bit 🤗

I am missing examples around:

• Defining basic metric types (and vectors). Only Histogram and Summary and collectors are there.
• Some summaries could be replaced by native histograms e.g.

  client_golang/examples/random/main.go

  Line 43 in b804be1

  rpcDurations: prometheus.NewSummaryVec(

• Testing instrumentation

Help wanted!

[keremgocen]

I want to start looking into this, which might take some time but please 🐻 with me as I do!

[stale]

Hello 👋 Looks like there was no activity on this issue for the last 3 months. Do you mind updating us on the status? Is this still reproducible or needed? If yes, just comment on this PR or push a commit. Thanks! 🤗
If there will be no activity in the next 4 weeks, this issue will be closed (we can always reopen an issue if we need!).