[dbnode] - Add filesystem available space reporting

[shrek]

What this PR does / why we need it:
Fixes #3400
Please refer to the discussion in the issue. In summary, this PR adds metrics
for available space in the filesystem, and the sizes of files in different m3db
directories. It can be used to alert on filesystem available space thresholds.
The added metrics look like this:

# TYPE filesystem_avail_bytes gauge
filesystem_avail_bytes 4.2259341312e+10
# HELP filesystem_commitlogs_bytes filesystem_commitlogs_bytes gauge
# TYPE filesystem_commitlogs_bytes gauge
filesystem_commitlogs_bytes 40
# HELP filesystem_data_bytes filesystem_data_bytes gauge
# TYPE filesystem_data_bytes gauge
filesystem_data_bytes 66304
# HELP filesystem_index_bytes filesystem_index_bytes gauge
# TYPE filesystem_index_bytes gauge
filesystem_index_bytes 4310
# HELP filesystem_num_dir_api_errors filesystem_num_dir_api_errors counter
# TYPE filesystem_num_dir_api_errors counter
filesystem_num_dir_api_errors 1.829242e+06
# HELP filesystem_num_fs_api_errors filesystem_num_fs_api_errors counter
# TYPE filesystem_num_fs_api_errors counter
filesystem_num_fs_api_errors 0
# HELP filesystem_snapshots_bytes filesystem_snapshots_bytes gauge
# TYPE filesystem_snapshots_bytes gauge
filesystem_snapshots_bytes 1.8173384e+07
# HELP filesystem_total_bytes filesystem_total_bytes gauge
# TYPE filesystem_total_bytes gauge
filesystem_total_bytes 6.2725623808e+10

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 89.85507% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.4%. Comparing base (1843061) to head (837f895).
⚠️ Report is 665 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (1843061) and HEAD (837f895). Click for more details.

HEAD has 84 uploads less than BASE

Flag	BASE (1843061)	HEAD (837f895)
collector	20	9
aggregator	22	9
m3em	20	9
cluster	20	9
msg	6	4
dbnode	17	9
query	15	5
x	11	5
m3ninx	16	5
metrics	5	4

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #3450     +/-   ##
=========================================
- Coverage    72.4%    64.4%   -8.0%     
=========================================
  Files        1100     1100             
  Lines      102443   101703    -740     
=========================================
- Hits        74203    65579   -8624     
- Misses      23143    31312   +8169     
+ Partials     5097     4812    -285     

Flag	Coverage Δ
aggregator	69.6% <ø> (-7.3%)	⬇️
cluster	65.7% <ø> (-19.2%)	⬇️
collector	84.3% <ø> (ø)
dbnode	75.6% <71.4%> (-3.4%)	⬇️
m3em	68.6% <ø> (-5.8%)	⬇️
m3ninx	62.8% <ø> (-10.8%)	⬇️
metrics	19.8% <ø> (ø)
msg	74.4% <ø> (+<0.1%)	⬆️
query	52.5% <ø> (-14.6%)	⬇️
x	63.3% <91.9%> (-17.3%)	⬇️

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