btc: add metric that reports account balance

[t-bast]

We can then easily alert on that metric when the fossilizer's balance is getting low. It will prevent us from being unable to fossilize important data (we'll need to send more btc to the fossilizer address which isn't automated yet though).

Resolves #500

---

This change is 

[codecov[bot]]

Codecov Report

Merging #501 into master will increase coverage by 0.04%. The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
+ Coverage   71.42%   71.46%   +0.04%     
==========================================
  Files          99       99              
  Lines        5627     5643      +16     
==========================================
+ Hits         4019     4033      +14     
- Misses       1220     1221       +1     
- Partials      388      389       +1

Impacted Files	Coverage Δ
blockchain/btc/blockcypher/blockcypher.go	66.17% <100%> (+1.02%)	:arrow_up:
blockchain/btc/blockcypher/metrics.go	89.47% <85.71%> (-1.23%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 7b19909...7939935. Read the comment docs.

[codecov[bot]]

Codecov Report

Merging #501 into master will increase coverage by 0.04%. The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
+ Coverage   71.42%   71.46%   +0.04%     
==========================================
  Files          99       99              
  Lines        5627     5643      +16     
==========================================
+ Hits         4019     4033      +14     
- Misses       1220     1221       +1     
- Partials      388      389       +1

Impacted Files	Coverage Δ
blockchain/btc/blockcypher/blockcypher.go	66.17% <100%> (+1.02%)	:arrow_up:
blockchain/btc/blockcypher/metrics.go	89.47% <85.71%> (-1.23%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 7b19909...7939935. Read the comment docs.