[prometheus-thanos] Allow for Thanos Receiver service to be headless or not

[tdinucci]

Signed-off-by: Tony Di Nucci tony.dinucci@skyscanner.net

What this PR does / why we need it:

Some systems (e.g. old versions of Istio) do not like headless services.

This PR updates the chart so that headless services are still the default for Receivers (so this is not a breaking change) however you can now configure things so that a standard service is created if you want.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] DCO signed
• [x] Chart Version bumped (if the pr is an update to an existing chart)
• [x] Variables are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [fluentd-elasticsearch])

[stale[bot]]

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