Add custom metrics

[alexellis]

Signed-off-by: Alex Ellis alexellis2@gmail.com

Description

Custom metrics are added for HTTP traffic and exposed by default on port 8081. This has been tested more extensively in the classic watchdog and copied / duplicated into the of-watchdog.

Motivation and Context

• [x] I have raised an issue to propose this change (required)

Several issues exist and a PR for the classic watchdog which was merged: https://github.com/openfaas/faas/pull/1151

How Has This Been Tested?

Local e2e testing was done on MacOS outside of a container and the new http_* fields and metrics were reported as expected.

https://github.com/openfaas/faas/pull/1151

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.