Add cmake support for prometheus exporter

[StevenYCChou]

To solve #248

[g-easy]

This looks great!

It builds and passes tests.

I tried to add the prometheus exporter to helloworld but we're missing exposer.h from prometheus-cpp::pull. You can see my branch here: https://github.com/g-easy/opencensus-cpp/commits/pr249

Would you like to handle pull as part of this PR, or in a followup?

[g-easy]

Also, please run tools/format.sh :)

[StevenYCChou]

Let's create another PR as this PR is self-contained. However, I identified a line too long from your commit, so I will push another commit to make the format right.

[g-easy]

tools/format.sh caught the long line ;)

Thanks for taking care of it.

[g-easy]

Agreed, let's leave prometheus-cpp::pull to another PR. Would you like to address the other comments in this PR?

[StevenYCChou]

I will address the issue you raise and follow up with you later - just saw your comments after I pushed that commit.

[StevenYCChou]

Do I need to rebase from master, or merge the latest branch from master? Or are you able to squash & merge them into single commit? I believe there won't be conflict.

[g-easy]

We always squash in opencensus-cpp.

Please:

• Update branch.
• Click "Squash and merge."
• Please clean up the commit message from github's suggestion before completing the merge.

Thanks :)

[StevenYCChou]

As I don't have write access, so I cannot merge back to master.
I will leave the commit message in my mind here for squashed commit and I hope it's useful for you:

Add cmake support for prometheus exporter.

This resolved only part of #249, because the 
build rule for prometheus_test_server is not added yet.  

[g-easy]

Thanks!