new(tests/e2e): add a metrics checker on e2e tests.

[FedeDP]

Fixes #288

Description

Implement a metricschecker for e2e tests.
It uses http GET calls to retrieve metrics from all tetragon pods. Note that i keep it simple and:

• did not leverage existing multiplexer package since it seems built around grpc connections and didn't want to add a whole new implementation. Also, since we are just using http.Gets it is really not needed imho.
• it is intended to be used as a last step in the test (before uninstalling the deployment) to check that desired metric has a certain value
• for now, i did not cache the metrics, thus any invocation of Great("mymetric", 0) will make the GET request to each metrics endpoint again
• Also, i didn't add support to filter the metric by labels