Promtail: add consul agent service discovery (#3834)

* working prototype

Signed-off-by: Trevor Whitney <trevorjwhitney@gmail.com>

* update test for new responses and metadata labels

Signed-off-by: Trevor Whitney <trevorjwhitney@gmail.com>

* fix linting issues

Signed-off-by: Trevor Whitney <trevorjwhitney@gmail.com>

* clarify license/copyright/attribution to prometheus code

Signed-off-by: Trevor Whitney <trevorjwhitney@gmail.com>

* undo changes to go.sum when adding consul dep

Signed-off-by: Trevor Whitney <trevorjwhitney@gmail.com>

* add documentation around consul and consulagent scrape configs

Signed-off-by: Trevor Whitney <trevorjwhitney@gmail.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

* Update docs/sources/clients/promtail/configuration.md

Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>

Co-authored-by: Ed Welch <edward.welch@grafana.com>
Co-authored-by: Karen Miller <84039272+KMiller-Grafana@users.noreply.github.com>