Feature: Prometheus Metrics Framework

[celestialorb]

Changes proposed in this PR:

This is a revamp of an old PR (#479) of mine. I'm getting back into KSP and would like a server that can export metrics in a way that Prometheus can scrape and capture them, allowing me to easily view trends over time in something like Grafana.

I decided to create a new branch and a new PR to avoid cleaning up the old one and to refresh my own memory as to how this works.

This initial PR just establishes a basic endpoint for Prometheus metrics and only exposes two metrics: one for total online player count as well as one for online status of individual players (if detailed player metrics are enabled).

If this is accepted I will create PRs to add individual groups of metrics to the server.
Looking at what I've done in the past for an old server of mine this would/could include:

• Space Program Metrics
  • Funds
  • Science
  • Reputation
• Vessel Metrics
  • Position
  • Orbital Elements
  • Staging
  • Resource Levels
• Facility Metrics
  • Levels
  • maybe damage?
• Contract Metrics
  • State of available contracts (i.e. rewards, accepted/offered)
• Technology Metrics
  • Science cost
  • Number of parts
• Subspace Metrics
  • Time difference of each subspace from server
• Kerbal Metrics
  • Attributes (bravery, dumb, etc.)
  • Flight Counts
  • Deaths
  • Recoveries

[celestialorb]

Tagging @gavazquez and @DasSkelett as they were involved in the review of the original PR.

[celestialorb]

Looks like the Appveyor jobs are failing. I guess the secret needs to be updated?

[DasSkelett]

Sorry for the long wait. Thanks for the work, looks very good, two small comments:

[celestialorb]

Added the changes recommended @DasSkelett! Performed some basic smoke tests with the changes and everything still works.

[celestialorb]

@DasSkelett Shall this be merged in, or would you like to fix the AppVeyor build first?

[celestialorb]

@DasSkelett Just a friendly poke on this PR.