Loki output does not contain custom headers defined via env variable

[lsroe]

Describe the bug

Custom headers for the Loki output set via LOKI_CUSTOMHEADERS headers are not added to the http/s requests.

How to reproduce it

Verify via GODEBUG=http2debug=2 and LOKI_CUSTOMHEADERS="Header: Token 1234" on falcosidekick side (header not added):

2025/01/29 10:32:15 http2: Transport encoding header ":authority" = "localhost:9988"
2025/01/29 10:32:15 http2: Transport encoding header ":method" = "POST"
2025/01/29 10:32:15 http2: Transport encoding header ":path" = "/loki/api/v1/push"
2025/01/29 10:32:15 http2: Transport encoding header ":scheme" = "https"
2025/01/29 10:32:15 http2: Transport encoding header "content-type" = "application/json"
2025/01/29 10:32:15 http2: Transport encoding header "user-agent" = "Falcosidekick"
2025/01/29 10:32:15 http2: Transport encoding header "content-length" = "290"
2025/01/29 10:32:15 http2: Transport encoding header "accept-encoding" = "gzip"
2025/01/29 10:32:15 http2: Framer 0xc000916000: wrote HEADERS flags=END_HEADERS stream=1 len=63

And on (Loki) server side:

--------------------Header------------------------
Content-Type        : [application/json]
User-Agent          : [Falcosidekick]
Content-Length      : [290]
Accept-Encoding     : [gzip]
-------------------------------------------------

Expected behaviour

Expected behaviour using Fix: #1107

2025/01/29 10:37:59 http2: Transport encoding header ":authority" = "localhost:9988"
2025/01/29 10:37:59 http2: Transport encoding header ":method" = "POST"
2025/01/29 10:37:59 http2: Transport encoding header ":path" = "/loki/api/v1/push"
2025/01/29 10:37:59 http2: Transport encoding header ":scheme" = "https"
2025/01/29 10:37:59 http2: Transport encoding header "user-agent" = "Falcosidekick"
2025/01/29 10:37:59 http2: Transport encoding header "header" = " Token 1234"
2025/01/29 10:37:59 http2: Transport encoding header "content-type" = "application/json"
2025/01/29 10:37:59 http2: Transport encoding header "content-length" = "290"
2025/01/29 10:37:59 http2: Transport encoding header "accept-encoding" = "gzip"
2025/01/29 10:37:59 http2: Framer 0xc00097c000: wrote HEADERS flags=END_HEADERS stream=1 len=79

Server side:

--------------------Header------------------------
User-Agent          : [Falcosidekick]
Header              : [ Token 1234]
Content-Type        : [application/json]
Content-Length      : [290]
Accept-Encoding     : [gzip]
-------------------------------------------------

Screenshots

Environment

Build from source at 58ec197423ddd5ab51c5049f854c67dfad424dfa

• Falco version:

• System info:

• Cloud provider or hardware configuration:
• OS:

• Kernel:

• Installation method:

Additional context

[Issif]

Nice catch, thank you for the PR too.