Add metrics api

[malteish]

First step of enabling metrics collection.

Changes:

• introduce 2 new values to delivery service properties: metrics collection interval duration and metrics retention duration
• count messages received by ds, including size.
• count notifications sent by ds
• make counted values available at the /metrics endpoint: https://testing.dm3.network/ds/metrics
• censor the current collection interval's values to improve privacy

[malteish]

I looked into the overhead of the function:

db.countMessage took 4.27 milliseconds, messageProcessor took 390.64 milliseconds. Total time: 394.9124559999909 milliseconds
db.countMessage took 2.14 milliseconds, messageProcessor took 337.57 milliseconds. Total time: 339.70516799998586 milliseconds
db.countMessage took 2.40 milliseconds, messageProcessor took 195.36 milliseconds. Total time: 197.76026800001273 milliseconds
db.countMessage took 1.96 milliseconds, messageProcessor took 456.14 milliseconds. Total time: 458.1027860000031 milliseconds
db.countMessage took 1.88 milliseconds, messageProcessor took 326.22 milliseconds. Total time: 328.10131399999955 milliseconds
db.countMessage took 1.44 milliseconds, messageProcessor took 324.39 milliseconds. Total time: 325.8282449999824 milliseconds
db.countMessage took 1.46 milliseconds, messageProcessor took 397.84 milliseconds. Total time: 399.30192700002226 milliseconds
db.countMessage took 1.68 milliseconds, messageProcessor took 345.11 milliseconds. Total time: 346.79461799998535 milliseconds
db.countMessage took 2.28 milliseconds, messageProcessor took 379.97 milliseconds. Total time: 382.24883900000714 milliseconds
db.countMessage took 1.54 milliseconds, messageProcessor took 357.71 milliseconds. Total time: 359.24879699997837 milliseconds
db.countMessage took 1.66 milliseconds, messageProcessor took 304.92 milliseconds. Total time: 306.58208699998795 milliseconds
db.countMessage took 1.26 milliseconds, messageProcessor took 325.23 milliseconds. Total time: 326.48554900000454 milliseconds
db.countMessage took 1.71 milliseconds, messageProcessor took 184.28 milliseconds. Total time: 185.994085000013 milliseconds
db.countMessage took 1.90 milliseconds, messageProcessor took 316.07 milliseconds. Total time: 317.9682070000272 milliseconds
db.countMessage took 2.21 milliseconds, messageProcessor took 357.79 milliseconds. Total time: 360.0047370000102 milliseconds
db.countMessage took 2.50 milliseconds, messageProcessor took 195.12 milliseconds. Total time: 197.61943799999426 milliseconds
db.countMessage took 3.47 milliseconds, messageProcessor took 342.11 milliseconds. Total time: 345.57568299997365 milliseconds
db.countMessage took 3.74 milliseconds, messageProcessor took 223.62 milliseconds. Total time: 227.3596560000151 milliseconds

happy with that, will keep it as is

[AlexNi245]

lgtm