Collect k8s endpoints, ingress, and services in kube_inventory plugin

[glinton]

Closes #5822

Adds support for collecting ingress, endpoint, and service information from kubernetes.

A few questions regarding what to collect:

• serivces: do we wish to add external_ips to the metrics
• endpoints: do we wish to add hostnames/ready state to the metrics
• ingress: do we wish to add ips to the metrics

Each of these additions would replicate the current metrics for each ip/hostname.

- kubernetes_endpoint,name=test,namespace=default,port_name=server,port_protocol=TCP created=1544102222000000000i,generation=0i,port=1234i 1562790821000000000
+ kubernetes_endpoint,hostname=127.0.0.1,name=storage,namespace=default,port_name=server,port_protocol=TCP created=1544102222000000000i,generation=0i,port=1234i,ready=true 1562790821000000000

- kubernetes_ingress,backend_service_name=test,name=test,namespace=default,path=/ backend_service_port=1234i,created=1544103262000000000i,generation=26i 1562790821000000000
+ kubernetes_ingress,backend_service_name=test,ip=1.0.0.127,name=test,namespace=default,path=/ backend_service_port=1234i,created=1544103262000000000i,generation=26i 1562790821000000000

- kubernetes_service,cluster_ip=127.0.0.1,name=test,namespace=default,port_name=diagnostic,port_protocol=TCP created=1551199644000000000i,generation=0i,port=1234i 1562790821000000000
+ kubernetes_service,ip=1.0.0.1,cluster_ip=127.0.0.1,name=test,namespace=default,port_name=diagnostic,port_protocol=TCP created=1551199644000000000i,generation=0i,port=1234i 1562790821000000000

Required for all PRs:

• Associated README.md updated.
• Has appropriate unit tests.

[glinton]

Test with the build that matches your platform:

• linux/

  • amd64

    • telegraf
    • telegraf-1.12.0~8cd7707d-0.x86_64.rpm
    • telegraf-1.12.0~8cd7707d_linux_amd64.tar.gz
    • telegraf_1.12.0~8cd7707d-0_amd64.deb
  • arm64

    • telegraf
    • telegraf-1.12.0~8cd7707d-0.arm64.rpm
    • telegraf-1.12.0~8cd7707d_linux_arm64.tar.gz
    • telegraf_1.12.0~8cd7707d-0_arm64.deb
  • armel

    • telegraf
    • telegraf-1.12.0~8cd7707d-0.armel.rpm
    • telegraf-1.12.0~8cd7707d_linux_armel.tar.gz
    • telegraf_1.12.0~8cd7707d-0_armel.deb
  • armhf

    • telegraf
    • telegraf-1.12.0~8cd7707d-0.armv6hl.rpm
    • telegraf-1.12.0~8cd7707d_linux_armhf.tar.gz
    • telegraf_1.12.0~8cd7707d-0_armhf.deb
  • i386

    • telegraf
    • telegraf-1.12.0~8cd7707d-0.i386.rpm
    • telegraf-1.12.0~8cd7707d_linux_i386.tar.gz
    • telegraf_1.12.0~8cd7707d-0_i386.deb
  • mipsel

    • telegraf
    • telegraf-1.12.0~8cd7707d_linux_mipsel.tar.gz
    • telegraf_1.12.0~8cd7707d-0_mipsel.deb
  • s390x

    • telegraf
    • telegraf-1.12.0~8cd7707d-0.s390x.rpm
    • telegraf-1.12.0~8cd7707d_linux_s390x.tar.gz
    • telegraf_1.12.0~8cd7707d-0_s390x.deb
  • static_amd64

    • telegraf
    • telegraf-1.12.0~8cd7707d-static_linux_amd64.tar.gz
• windows/

  • i386

    • telegraf-1.12.0~8cd7707d_windows_i386.zip
    • telegraf.exe
  • amd64

    • telegraf-1.12.0~8cd7707d_windows_amd64.zip
    • telegraf.exe
• freebsd/

  • i386

    • telegraf
    • telegraf-1.12.0~8cd7707d_freebsd_i386.tar.gz
  • amd64

    • telegraf
    • telegraf-1.12.0~8cd7707d_freebsd_amd64.tar.gz

[danielnelson]

I would try to avoid adding the IPs, do we have enough information to omit them and still have uniqueness? Is there a dns name that could be used instead? Hostnames on the other hand are probably good to collect, and the ready flag also seems useful (so long as there are only two states).