feat/target-manager-registered-metric

[puffitos]

Motivation

Part of #231

A metric in the target manager component should expose the current state of the sparrow (whether it's registered or not.)

Changes

• Added metrics to target manager
• Added new gauge metric (0/1) to the targetmanager, to expose the registration status
• Set the new metric to 0/1 upon successful registration & deregistration of sparrow.

Tests done

Manual E2E tests were done. The sparrow exposes the new metric and it's 0 on start and is set to 1 upon registration.

❯ curl localhost:8080/metrics | rg register
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 12718    0 12718    0     0  8112k      0 --:--:-- --:--:-- --:--:-- 12.1M
# HELP sparrow_target_manager_registered Indicates whether the instance is registered as a global target
# TYPE sparrow_target_manager_registered gauge
sparrow_target_manager_registered 1
❯ curl localhost:8080/metrics | rg register
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  9743    0  9743    0     0  6859k      0 --:--:-- --:--:-- --:--:-- 9514k
# HELP sparrow_target_manager_registered Indicates whether the instance is registered as a global target
# TYPE sparrow_target_manager_registered gauge
sparrow_target_manager_registered 0

Deregistration wasn't tested in an e2e setting, as deregistration only happens upon shutdown -> the metric isn't exposed anymore. I didn't start a local prometheus just to check whether the last state would've parsed. Additionally, as there is no guarantee that the last state of the metric will be parsed from prometheus, I don't know how one can easily test this scenario without some complicated setup :)

• Unit tests succeeded
• E2E tests succeeded

TODO

• I've assigned this PR to myself
• I've labeled this PR correctly