* updated CustomHandler interface
 * unified how query routes are added. no need to pass router around and wrapping is done in one place.
 * routes are registered with names so they could be easily found later when custom handlers are registered.

* master: (28 commits)
  [dbnode] Add claims for index segments volume index (#2846)
  [dbnode] Remove namespaces from example config and integration tests (#2866)
  [dbnode] Resurrect flaky test skip (#2868)
  [aggregator] Fix checkCampaignStateLoop (#2867)
  [dbnode] implement deletion method in namespace kvadmin service (#2861)
  Replace closer with resource package (#2864)
  Add coding style guide (#2831)
  Add GOVERNANCE.md to describe governance (#2830)
  Add COMPATIBILITY.md to describe version compatibility (#2829)
  Refactor etcd config as discovery section with convenience types (#2843)
  Refactor x/lockfile into dbnode/server (#2862)
  [lint] Disable nlreturn linter (#2865)
  [m3cluster] Expose placement algorithm in placement service (#2858)
  [etcd] Set reasonable cluster connection/sync settings by default (#2860)
  [dbnode] Use bits.LeadingZeros64 to improve encoder performance (#2857)
  Cleanup m3nsch leftovers (#2856)
  Update ci-scripts to correct coverage tracking (#2854)
  [aggregator] Process campaign state without waiting for first campaign check interval (#2855)
  Bump go to 1.14 (#2853)
  [query] Remove single series error from M3
  ...

…ing for named route.

…nly used "namespace" label (#2878)

* [coordinator] Set default namespace tag to avoid colliding with common "namespace" default value

* Use defined constant

* Add downsampler test case to demonstrate override namespace tag

Co-authored-by: Wesley Kim <wesley@chronosphere.io>

* Improe some slow tests

* lint

* lint

* goddamn imports

* master:
  [query] Remove dead code in prom package (#2871)
  Improve some slow tests (#2881)
  [coordinator] Set default namespace tag to avoid colliding with commonly used "namespace" label (#2878)
  [dbnode] Introduce Aggregator type (#2840)
  [dbnode] Refactor wide query path (#2826)
  [dbnode] Add validation to ensure index claims is singleton (#2877)
  [dbnode] Default HostID and Discovery config for minimal config (#2876)

Co-authored-by: Vilius Pranckaitis <vpranckaitis@gmail.com>

Co-authored-by: Vilius Pranckaitis <vpranckaitis@gmail.com>