Add event and service check metric type

[fmejia97]

This PR provides the same functionality as #109 , but without the refactor. The aim is to see if we can reduce the 16% performance hit from the previous PR.

[fmejia97]

Benchmarking results

TLDR; Performance hit is now 42%

Results for code with NEW changes

increment metric benchmark
                        16.548k i/100ms
measure metric benchmark
                        13.180k i/100ms
gauge metric benchmark
                        17.076k i/100ms
set metric benchmark    13.505k i/100ms
Calculating -------------------------------------
increment metric benchmark
                        193.313k (±10.6%) i/s -    959.784k in   5.031107s
measure metric benchmark
                        159.501k (± 8.0%) i/s -    803.980k in   5.073448s
gauge metric benchmark
                        222.199k (± 4.9%) i/s -      1.110M in   5.007758s
set metric benchmark    145.680k (± 9.2%) i/s -    729.270k in   5.047882s 

Results for current master

Warming up --------------------------------------
increment metric benchmark
                        28.004k i/100ms
measure metric benchmark
                        21.855k i/100ms
gauge metric benchmark
                        29.549k i/100ms
set metric benchmark    20.899k i/100ms
Calculating -------------------------------------
increment metric benchmark
                        283.951k (± 8.9%) i/s -      1.428M in   5.067626s
measure metric benchmark
                        231.504k (± 6.1%) i/s -      1.158M in   5.022373s
gauge metric benchmark
                        303.683k (± 8.0%) i/s -      1.537M in   5.092620s
set metric benchmark    200.402k (± 9.9%) i/s -      1.003M in   5.065671s