Document exported metrics in README

[gebn]

With examples, e.g.

Power draw by manufacturer and model:

sum by (manufacturer, model) (chassis_power_draw_watts)

Number of machines by BMC firmware:

sum by (firmware) (bmc_info)

Number of machines with a cooling fault:

count(chassis_cooling_fault == 1)

Ratio of machines currently powered on:

count(chassis_powered_on == 1) / count(chassis_powered_on)

[gebn]

The scrape output would be helpful for showing what's available.

[gebn]

# HELP bmc_info Provides the BMC's GUID, firmware, and the version of IPMI used to scrape it. Constant 1.
# TYPE bmc_info gauge
bmc_info{firmware="3.45.01",guid="04d298z2-8178-11e5-adf4-54ab3a0a0baa",ipmi="2.0"} 1
# HELP bmc_last_scrape When the BMC was last scraped by this exporter, expressed as seconds since the Unix epoch. This metric will not be present in the first scrape of a given BMC, including after GC.
# TYPE bmc_last_scrape gauge
bmc_last_scrape 1.56674752208897e+09
# HELP bmc_scrape_duration_seconds The time taken to collect all metrics, measured by the exporter.
# TYPE bmc_scrape_duration_seconds gauge
bmc_scrape_duration_seconds 0.018077478
# HELP bmc_up 1 if the exporter was able to gather all desired metrics this scrape, 0 otherwise.
# TYPE bmc_up gauge
bmc_up 1
# HELP chassis_cooling_fault Whether a cooling or fan fault has been detected, according to Get Chassis Status.
# TYPE chassis_cooling_fault gauge
chassis_cooling_fault 0
# HELP chassis_drive_fault Whether a disk drive in the system is faulty, according to Get Chassis Status.
# TYPE chassis_drive_fault gauge
chassis_drive_fault 0
# HELP chassis_intrusion Whether the system cover is open, according to Get Chassis Status.
# TYPE chassis_intrusion gauge
chassis_intrusion 0
# HELP chassis_power_draw_watts The instantaneous amount of electricity being used by the machine.
# TYPE chassis_power_draw_watts gauge
chassis_power_draw_watts 245
# HELP chassis_power_fault Whether a fault has been detected in the main power subsystem, according to Get Chassis Status.
# TYPE chassis_power_fault gauge
chassis_power_fault 0
# HELP chassis_powered_on Whether the system is currently turned on, according to Get Chassis Status. If 0, the system could be in S4/S5, or mechanical off.
# TYPE chassis_powered_on gauge
chassis_powered_on 1