postgres_exporter/percona_tests

integration tests for exporter update

Fast start:

run

make prepare-env-from-repo

then run any of the make test-*

A bit of details:

1. unpack original exporter

   make prepare-base-exporter

2.a. download updated exporter from specific feature build

make prepare-exporter-from-fb url="<feature build client binary url>"

2.b. or use current repo as updated exporter

make prepare-exporter-from-repo

3. start test postgres_server

   make start-postgres-db

4. run basic performance comparison test

   make test-performance

5. run metrics list compatibility test

   make test-metrics