Add Jaeger distributed tracing support

[leth]

Allows trace reporting to be configured via environment variables e.g. JAEGER_AGENT_HOST, JAEGER_SAMPLER_TYPE.

[bboreham]

LGTM, thanks

[satyamz]

lgtm!

[bboreham]

List of changes pulled in from weaveworks/common:

• Don't use full URL as metric label on unmatched path (Don't use full URL as metric label on unmatched path common#108)
• Raise default server timeouts from 5s to 30s (Raise default server timeouts from 5s to 30s common#109)
• Return errors when http/grpc servers exit. (Return errors when http/grpc servers exit. common#92)
• Expose jaeger metrics (Expose jaeger metrics common#114)
• Allow the AWS backend to get credentials from its environment. (Allow the AWS backend to get credentials from its environment. common#111)