Add Prometheus metrics to Connector (Fixes #146)

[sappenin]

Fixes #146

• Adds prometheus metrics tracking for incoming and outgoing packets.
• Adds Link and PacketSwitch filter unit tests.

[lgtm-com]

This pull request fixes 1 alert when merging a20ae31 into 9c5f4a2 - view on LGTM.com

fixed alerts:

• 1 for Equals on incomparable types

[codecov]

Codecov Report

Merging #392 into master will increase coverage by 2.32%.
The diff coverage is 78.24%.

@@             Coverage Diff              @@
##             master     #392      +/-   ##
============================================
+ Coverage     52.63%   54.95%   +2.32%     
- Complexity      655      693      +38     
============================================
  Files           231      239       +8     
  Lines          4334     4595     +261     
  Branches        185      185              
============================================
+ Hits           2281     2525     +244     
- Misses         1983     1999      +16     
- Partials         70       71       +1

Impacted Files	Coverage Δ	Complexity Δ
...ledger/connector/balances/RedisBalanceTracker.java	80.18% <ø> (ø)	17 <0> (ø)	⬇️
...r/connector/link/http/AbstractBlastHttpSender.java	0% <ø> (ø)	0 <0> (ø)	⬇️
.../connector/core/settlement/SettlementQuantity.java	100% <ø> (ø)	4 <0> (ø)	⬇️
...erledger/connector/config/CaffeineCacheConfig.java	0% <0%> (ø)	0 <0> (ø)	⬇️
.../interledger/connector/metrics/MetricsService.java	0% <0%> (ø)	0 <0> (?)
...ector/events/IncomingSettlementSucceededEvent.java	0% <0%> (ø)	0 <0> (?)
...onnector/events/IncomingSettlementFailedEvent.java	0% <0%> (ø)	0 <0> (?)
...ledger/connector/link/http/JwtBlastHttpSender.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ts/OutgoingSettlementInitiationSucceededEvent.java	0% <0%> (ø)	0 <0> (?)
...vents/OutgoingSettlementInitiationFailedEvent.java	0% <0%> (ø)	0 <0> (?)
... and 30 more

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[lgtm-com]

This pull request fixes 1 alert when merging aed7eea into 9c5f4a2 - view on LGTM.com

fixed alerts:

• 1 for Equals on incomparable types

[lgtm-com]

This pull request fixes 1 alert when merging 8a56ddd into 9c5f4a2 - view on LGTM.com

fixed alerts:

• 1 for Equals on incomparable types

[lgtm-com]

This pull request fixes 1 alert when merging a35e743 into 1cbd0a8 - view on LGTM.com

fixed alerts:

• 1 for Equals on incomparable types

[lgtm-com]

This pull request fixes 1 alert when merging 00bd178 into 1cbd0a8 - view on LGTM.com

fixed alerts:

• 1 for Equals on incomparable types

[theotherian]

Overall this looks good to me; just the one comment to consider on CryptoCompareRateProvider.

[lgtm-com]

This pull request fixes 1 alert when merging 9927db2 into 1cbd0a8 - view on LGTM.com

fixed alerts:

• 1 for Equals on incomparable types