Add feature to filter project ID at call-time

[xairos]

Hey @SuperQ ! This is an implementation of the feature proposal in #145, TLDR to allow filtering by project ID at scrape-time. Our use case is multi-tenancy, where each tenant can scrape a centrally hosted SD exporter with their own project IDs as scrape parameters.

Closes #145.

Breaking changes

Call-time filtering via the collect parameter now also matches if any preconfigured metric prefix is a string prefix for the call-time filter.

Example

Running stackdriver exporter with the prefix dataflow.googleapis.com/job/ would allow you to use the following collect config:

collect:
- dataflow.googleapis.com/job/is_failed
- dataflow.googleapis.com/job/elapsed_time