Allow default per-operation sampling strategies

[rutgerbrf]

Requirement - what kind of business use case are you trying to solve?

The /health and /metrics endpoint make other traces hard to find.

Problem - what in Jaeger blocks you from solving the requirement?

These traces can be ignored with the fix of #814. This fix allows to set operation strategies on a per-service basis, but with a lot of services it can quickly become unmaintainable to disable spans for the health endpoint for every service.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Allow to specify operation strategies in the default sampling strategy, like so:

{
  "default_strategy": {
    "type": "probabilistic",
    "param": 1,
    "operation_strategies": [
      {
        "operation": "op1",
        "type": "probabilistic",
        "param": 0.2
      },
      {
        "operation": "op2",
        "type": "probabilistic",
        "param": 0.4
      }
    ]
  }
}

Any open questions to address

None

Note: this issue is also very closely related to #1387