Add HealthCheck middleware

[JoelSpeed]

Description

Adds healthchecking handler as a middleware

Motivation and Context

Handle all healthchecking in one place. Allow testing of healthchecking separate from other components. Allow registering health check middleware before logging handler to implement silent logging.

How Has This Been Tested?

Unit testing and manual testing using local testing environment

Checklist:

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.

CC @chkohner @kskewes (I feel you may both have an interest in this)

[karlskewes]

Nice, this is much cleaner. LGTM.

To remove the need for GCP specific code I guess we would need to make opts.PingPath a slice of strings rather than a single string. That could always be done later if desired.

[JoelSpeed]

To remove the need for GCP specific code I guess we would need to make opts.PingPath a slice of strings rather than a single string. That could always be done later if desired.

I had imaged we just deprecate the options and say to people that they will have to migrate their apps to use a single health/readiness endpoint, it shouldn't be very difficult I wouldn't have thought

[JoelSpeed]

Just did a bunch of manual testing of this and I'm happy that this is working as expected and retains all of the existing healthcheck behavour

[JoelSpeed]

Merging based on my own and @kskewes review, if anyone has any later comments I'll address those in a follow up